Nice challenge, is a challenge with only the two of you? Or is it a big competition?

I dont know about the order, but remember to push hard with your legs on the rowing machine! Most people will use their back-muscles, but your leg-muscles are much stronger!

I would do the rowing as last, because it will tire you out, you back, arms, legs....

You're both a experienced runner and biker so i would go for swim, bike, run and rowing. If necessary you can both slow or fast on the bike depending on your feeling when coming out of the water without being exhausted at the start of the running part. Rowing will tire you, how fit you are at the start, because you're not familiar with the technique. And you will use muscles you normally use less. As Aristos said, use your legs mostly. And extend your strokes by pulling your arms to your body when your legs are fully streched. Try to keep your back straight, leaning forward reduces the stroke length. I hope this short explanation helps. Probably seeing somebody rowing helps more. ;-)
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