I use a two person SPIN TWIN tarp, and it's pretty snug for two, and AWESOME for me when I'm alone. I'm tall, and it's plenty long, and plenty wide.
THat'll solve any issues of size.
And - I have a lot of tarp experience. If it rains, I find I do surprizingly well. One thing is that I spend a little time picking the perfect site (where a puddle won't form) and sheltered enough so I won't suffer from excess wind. That's pretty simple.
A synthetic bag might provide a little extra water resistance - but a down bag is fine. Adding a bivy is helpful.
And - BPL sells a twin bivy, and that would be a solution to a down bag, and it would nix the need for a ground-cloth. It's kind of expensive, so I can't truly recommend it. Actually, a big bivy sack would be nice, because they are all so dinky.
(Hmmmm - I bet if you wrote a nice letter, the folks at BPL might send it to you to test - before buying - maybe)
Also - The tarps are all open, so they are super as far as air flow, you'll be DRYER under a tarp than in a tent (really)!