Best way is to start off with buying a second Tm42. There isn't a single good* 2in1 manifold on the market.

*And with good I mean worth more than the scrap-value of the metal used.

Known configurations are dual Dellorto 40ies with velocity stacks and Mikuni VM38s sucking through the frame. Sepp has lots of experience with the first version, myself (for legal reasons as I live in Austria and not in Germany) prefer the VM38s. There's also a flatslide carb version of the VM38s, called the TM38 and I will start fooling around with that one rather soon, but I can't give you jetting advice as of yet, but in a few weeks I should probably be able to give you a baseline.

Hope this helps
Greg

P.S.: Both for the VM38 and Dellorto 40 there are detailed jetting and parts lists available on the forum.