[RTTY] Antenna Separation Question

[Don Hill AA5AU]

I know this is off topic from RTTY, but I trust the opinions of the subscribers
here.

I'm going to install a SteppIR 3 element yagi at 75' and install a 2 element
Force12 40 meter yagi above it.

What's the minimum distance I can get away with between the SteppIR and 40M
beam?  I'm trying to determine the length of mast I need.  

And I have an M2 rotor in excellent shape I'm going to use (pretty cool rotor
really).  How far down into the tower should I install the rotor?  I'm thinking
4 feet.

Thanks,
Don AA5AU