ICs on opposite sides of a PCB
If you are concerned about signal integrity then you most certainly want to have internal planes. Your best bet would be to have two internal ground planes to provide a low-impedance return path, as well as shielding between the chips. So to answer your question, no - there probably won't be an issue with putting one chip &quot;above&quot; another on the opposite side of the board provided you have internal ground planes.
When it comes to hot air soldering, yes - it will be more difficult to solder parts with a connection to a large mass of copper because said copper will wick away the heat more easily. A good hot air tool will be able to overcome this though.