Yes you can, but with limits.
We put a lot of 2-up miles on a Speedmaster until we moved up to a bigger cc machine. Once you're rolling you'll be fine but let's be honest - the 865 doesn't have the oomph that your 1600 has.
As far as weight, the America is a much lighter machine than the TBird, so you'll appreciate that. In the last 2 years we've put over 50,000 2-up miles on 800 pound Visions and I'll be the first to admit it's a lot of weight at stop. You'll either get used to it, or you won't.
Whatever you decide, good luck and ride safe.
