Pilot DAR,
You could do worse than inspect and fly a couple of Piper Twin Comanche's. Firstly, Try an unmodified PA 30, then try a PA 39. Reason I suggest this is, Piper had a go at producing a light twin with non-handed engines (PA30). Then later fitted counter-rotating engines (PA39). Still a number of these aircraft around, and the manufacturer put a fair bit of effort into making them both work properly. Probably the easiest and quickest way for you to determine what your project aircraft should 'feel' like, and what kind of basic handling differences exist. These aircraft types also have fairly comprehensive POH's.
The reason I suggest an unmodified PA30 is, a number have been retrofitted with counter-rotating engines by STC.
Hope this helps.
camlobe
P.S. How about letting us know how you get on?