Hey, congratulations to you and your friend for grabbing the reigns. Don't worry what others think, you'll make tons of new friends who think your interesting because you tandem together.
I'm 6' 2'' 205 , myself and have been as heavy as 250 (lbs). Even through my heaviest phase I was commuting and cycling for fun. For me, serious weight loss meant diet and bumping the bike (gradually) up to 300-400 miles/week. You burn 500-1000 cal/hour depending on effort level cycling, so it's among the most efficient calorie burners. But still, saying no to the fast-food lunch will knock down 1500 cal pretty easily.
Sorry, that's starting to run preachy, not my intent, just my own experience.
Anyway, if you get up into serious training level riding here are a few things a novice out of shape guy should probably be aware of or warned about:
1. Bonking. You're trying to lose weight. You cut calories, eat less. You go out and ride and feel great for an hour. You're miles from home. Suddenly you feel no so great. Your legs just can't be willed to give you power. You feel geenerally lousy and your head gets a little foggy. You've just bonked. Your body ran out of fuel. You need to eat a little as you ride. Have a banana or a granola bar every 45 minutes or hour. Drink lots of water. Keep a "Gu" pack on you in case it gets really bad.
2. Saddle sores. Pretty much explains it all. The funny lycra tights with what looks like a diaper pad are used for a reason. Sores happen but can be dealt with. Don't just "power through" buy don't let it put you off. Just deal with it. Balms, pads, tights, and most importantly the right saddle for you can make it all ok. Don't confuse saddle sores, a skin problem, with the soreness of your sit bones. Your sit bones will acclimate, that is, you should just power through that ache.
3. Have a reasonable train plan with attainable goals. Don't try to bump up you weekly milage more than 20-30 miles at a go and don't try to bump your one-day milage more than about 5 at a time. I like to ramp up by starting with a couple of short (meaning 30 miles for me, that may mean 10 miles starting for you) rides per week and add another day each week till I get to 5-6 days/wk. Then I add 5 here, 5 there, till I've got a couple of 80 mile days, a couple of 40 mile days and a day or 2 of just fun little sprints. And then the season is over and it's time to hybernate (just kidding). Have fast days and slow days. Days you stop and take pictures and picnic, and days you put the hammer down and go for a new speed record.
4. Remember that it's fun. If you dread riding, you'll stop. If it ceases to be fun, figure out why and fix that. maybe you pushed too hard, too far. Back off. Maybe you need an extra day off. Maybe you need to find a social group to ride with. Maybe you need to get away from the social group you're riding with. Just remember that fun is the incentive that will keep you getting on the bike long term.
5. Tires. Lots of opinions on this. My feeling is that if you're riding for fitness, pick a tire that is comfortable to ride and flat resistant. Flats are annoying especial for a novice who may or may not be comfortable fixing them. The whole point of an inflatable tire, as opposed to solid rubber, is that it provides pneumatic suspension. Rock-hard skinny tires tend to fatigue me on long rides. The transmission of road imperfections to your body takes a toll. I run 700x28mm on my single and 700x35 on my tandem. Just a personal choice, but there's my thinking.
Good luck guys.