How far should the two propellers on a twin engine plane be apart?
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.