I was wondering if there are any specific ways to calculate the appropriate distance between two propellers for a twin engine twin fuselage planes like a Twin Mustang plane?
I'm assuming that you want to minimize efficiency losses due to propellers competing for clean air. I do no know what the optimal distance is but looking at a few practical examples it seems that as long as the propeller disks aren't overlapping you should be alright.
Here is a B 29 superfortress front view you can see the propeller disks are tangential to each other. The B 17 has a similar set up.
Here is a Tupolov bear as a more modern example there is more space between the propeller disks here than on the b 29. I would say this is probably because of the significant difference in horse power between the bears engines and a b 29's.
I hope this helps shed some light on what you were looking for. If you need more information your best bet is to look for a NACA paper on the topic. If there is research on this (and published in english) they are likely to be the ones who have done some.
For twin fuselage aircraft, the propeller distance is not the only parameter to look out for.
Safety-wise, the distance of propellers affect the size of the vertical tail and rudder. Because, the aircraft must still be trimmable / flyable in the loss of one of the engines.
The fuselage separation affects the weight breakdown of the aircraft as a whole. It is actually a relief for the wing loads.
From the viewpoint of Propellers, there's almost no problem in bringing them in close proximity. They can even intermesh, as in the case of some helicopters. They will generate unwanted pressures to each other and extra noise, if they approach too much. However, if not overlapping, propellers do a good job of streamlining a flow (and adding a twist to it).