You need to buy a copy of EZ-Nec, you can model wire antenna to your hearts content.
You are worrying a little too much about antenna interaction. I usually just put new antennas up and see how they work. You can open and short the feedline of the new antenna while you are tansmitting on the old one and see if the SWR changes. That can indicate interaction. You can also look for effects in recieved signals.
If you still want to consider interaction I would use rules of thumb like keeping like polarized antennas at least 1/2 to 1 wavelength apart and mount horizontal antennas at right angles if possible.
But your best bet is an antenna analysis program. You will also learn a lot about antennas as well.
try this web site: http://www.cebik.com/