Ivarb, if you have not done a pull/pull before, I would wait for a fairly simple profile or stick where you have lots of freedom to improvise, rather than try to work out the complications on this model. I really would go for a simple solution. Simplest is to let the tailwheel caster freely. This is definitely the most reliable, foolproof solution as far as ground-handling and flying go.

If you feel you have to have a steerable tailwheel, the next simple solution is to have the tailwheel control arms stick out the sides of the fuse, stick a T-pin up into the bottom of the rudder and simply run rubber bands with light tension from control arms to the pin. Or springs instead of rubber band. If you want to buy another control horn just for the tailwheel, mount it opposite the pushrod control horn, and use inner holes on either side for the tailwheel springs.

I personally do not like the control horns that bolt through to a pad on the other side of the surface, although I admit they do work OK. My own preference is for Rocket City horns, now sold by Nelson Hobby. These use a threaded post and special pivot points. I run the post through a hole in a dowel that I glue into a hole in the control surface, giving a solid hard point and much better alignment than I used to get with nylon horns.

I'm not sure from your photo whether I'm looking at a bottom half shell of fuselage, which would permit adjustment of tailwheel controls inside the fuselage before gluing it all up forever. That might accommodate a more sophisticated solution, but if you are into sophisticated solutions you should first try all the simple solutions to convince yourself they don't work well (but they do).

To answer your question, I'm sure it would be possible, in principle anyway, to run two pushrods off of one servo wheel or arm, one to rudder and one to tailwheel. However I've never seen nor heard of such a rig on tailwheel, although it is conventional to control nosewheel this way. The disadvantage of direct pushrod to nose/tail-wheel is that every time the wheel hits a pebble the wheel takes a torsion whack which is transmitted via pushrod back to servo gears, causing premature wear and failure. There are gadgets to absorb these shocks, but they cost money, and in my view mainly serve to complicate a simple problem.