Are you hitting as hard as you can?" asks Duncan Allen - padded hands in front of his body as his girlfriend, Tanja Schubert, winds up to take another shot. You wouldn't guess it from the way Schubert throws her next, extra-hard punch, but these two are meant to be showcasing the wonders of working out as a couple. At the moment, though, they look more like an advertisement for some kind of aggression therapy.

Amelia Watts, a personal trainer for Benefit Personal Training, is introducing Schubert and Allen to training as a couple this afternoon. She has a growing list of clients who gain more than just physical benefits from working out as a pair. It's a practical solution - if, like Watts, your personal trainer charges by the hour rather than the person, what is unaffordable for one becomes reasonable for two. But there are other advantages to working out as a pair.

"I've had some couples," says Watts, "where there's been tension in the air when I arrive, but at the end of the session they're laughing because there has been a controlled release of aggression."

We've spent the afternoon in the park, with Allen and Schubert getting to grips with some exercises designed for couples. They both enjoy keeping fit, and joined a gym a couple of years ago, but "we would tend to do our own thing," says Schubert, "mainly because we don't know what to do together. "

This is where Watts comes in - working out exercises that involve teamwork and trust, and which are designed to make people work together. She puts Schubert and Allen through a circuit of five exercises that stretch the couple's communication and problem-solving skills as well as their physical ability.

In the longer term, Watts says, working out together also helps with motivation between personal-trainer sessions. "People do have days when they don't want to exercise - if you train in a couple, the other person can give you the motivation to go and do it. Also, if you're training with someone else, they are likely to push you more - it's almost like having a personal trainer there."

There are problems, of course. Schubert and Allen have a similar level of fitness - but that's not true for all couples, and making sure both partners get a decent workout isn't always straightforward. "I have trained couples who had completely different needs," Watts says. "I had to design separate workouts. But they did them at the same time, and still got the motivation. They warmed up and did the stretching at the end together as well. You can combine elements." While you might have differences in, say, strength, it might be that for cardiovascular work you're at a pretty similar level so you can run or box together, even if you work separately in other areas. In any case, keeping the advantage, or closing the fitness gap, gives both of you goals to aim for.

Which brings us neatly to one potential snag in the whole dual working-out dream: competition. While a little competitive spirit can be just what a couple needs, too much can add to the tension that exercise is meant to be helping dissipate. "Because exercise is seen as this masculine domain, men do think they should be better at it then their girlfriend," says Watts. "To a certain degree it's good because it might push you to do an extra five press-ups, but there are some couples who just can't train together and they work that out pretty quickly."

With Watts's words fresh in my mind, I wake up on Sunday morning to a hangover, a lot of rain, and a promise to play football with my boyfriend. Not every couple wants to work out with touchy, trusty exercises, so we're off to play a team game down the park. Or rather I'm going to learn how to kick a ball properly, and he's going to feel brilliant because he'll be able to do all his tricks and show off.

I'm slightly sceptical, not least because we get competitive over who does the best washing-up. Still, while we search for a bit of un-waterlogged pitch - one bit is so boggy it pulls off my trainer - I remember Watts's encouragement to treat your workout as a chance to spend quality time together. "So many people have no time to spend with each other," she says. "You both need to exercise, so if you exercise together you manage to get quality time and do everything you need to. Because you're being physical, it also means you're not spending the time talking about stress and work."

So off we go. I am rubbish, but at least my inability to kick straight/at a sensible height/in anything like the right direction means we both have to do a lot of running about just fetching the ball. And, astonishingly, despite being covered in mud, rain and possibly dog poo, we're having fun. Things are so jolly, we even brave the rain a bit longer to do some netball drills, which has the added benefit of turning the competency tables.

United by football. For me, a Mancunian living with a Liverpool fan, that really must count as something of a miracle.

www.benefitpersonaltraining.co.uk

It takes two: Four great partner workouts

Squats (for legs, hips and abs)

1 Stand back to back, feet slightly wider than shoulder-width apart and hands linked.

2 Draw in your abdominals and walk your feet forward slightly, so you lean against each other.

3 Using each other for support, bend knees and hips and lower towards the floor, until you have a 90-degree angle in your knees. Slowly push up to standing. Repeat 10-15 times.

Seated inner-thigh stretch

1 Sit facing each other with legs out wide. Your feet should be placed next to each other.

2 Keeping your back as straight as possible, lean forward and hold hands.

3 Partner one, gently pull partner two closer to increase the stretch. Hold for 30-60 seconds, relax and swap over.

Medicine ball oblique twists (for obliques, arms, legs)

1 Stand back to back, shoulders back, and abs pulled in.

2 Partner one: holding the ball at arm's length, chest height, twist to pass the ball to partner two - moving your arms from right to left.

3 Partner two: repeat the movement moving left to right. Do 10 reps then change sides.

Crunches with medicine ball throw

1 Lie on your backs, feet to feet, with legs bent.

2 Partner one: holding medicine ball overhead, crunch up - using the abs - throw the ball to your partner, and hold position.

3 Partner two: catch the ball, and slowly lower youself to the floor taking the ball overhead. Crunch up, throwing the ball to partner. Repeat 15-20 times.