Situation:
I am basing a SW J-37 Transport and Sgt Leo Reordan simultaneously with my Liao ATV squad. Can the opposing player make a ranged combat order against my ATV squad while it is simultaneously in base contact with both units?

Yes, I think. But better get a more reliable person to answer before running off and yelling at everyone who says I am wrong.

Only if the unit is in base contact with the ATV.

Riordan could probably not shoot at the ATV due to minimums (if based near the front, the minimum may be met to allow the shot). The J37 could shoot at the ATV. Anybody else could not shoot at him.

Are you talking about another (third unit) who is not in base contact? Like in Mk w/ precision? No. Not in MW. If you are in base contact you cannot be targeted by an outside unit w/a ranged combat order (unless it is artillery)

But if you mean one of the units you have based makes a ranged combat attack yes they can.