NO! NO! NO! WARNING, DO NOT DO THAT, EVER.
Speaker outputs are TOTALLY different than line/instrument level outputs, the 200s has NO slave out. NEVER run a tube amp without a load. It'll cook the output transformer in no time.
Always have a load on your amp.
Speaker outputs are in direct link to the output tubes, you'll be sending the full power of the amp into the low voltage instrument/line level inputs, and that will smoke the amp, coupled with having no load on the 200s, you'll cook it, too. NEVER, and I repeat NEVER, do that.
Solid state amps can run without a load attached, but I wouldn't suggest it. You can get two cabs, and use the preamp of the concert bass to slave the 200s, which is totally fine as long as the impedance matches with the 200s/cab.
If you want a tube preamp for the concert bass, give the English Muff'n a shot, it's cheap, and you won't tank both of your amps with it.