Build a Pitts S-1C. It is a very simple biplane and the design first flew in 1945. It flew with less than 65 hp. It is small (lo cost for materials), strong, simple, plans and parts are available, and builder support is available too.
And you will impress your friends when you fly it!
I'm not sure if 85hp is enough to drag two wings and two people around. I haven't seen it. I think you'll need to use a higer powered engine or, if you're stuck on 85hp max, give up a wing or seat. Good luck.
All the above answers are wasting your time (well meant though). For a real vintage looking 2 seat Biplane, look at the Flitzer Z-2 Swalbe.
A good info source is: Flitzer Sportflug Verein
You will find all your dreams met with the Flitzer, support is first class which is the most important thing after the design. Lynn Williams the designer is always on hand and goes out of his way to help and the builder group is second to none.
Look up the Hatz biplane. Just what you asked it looks like a classic biplane , 2 place. It would be my choice. One of them took a Grand Champion at Oshkosh a few years ago. I think they still do support and all. Check them out. Nice plane.