You might want to move your antenna's farther apart. I asked when I did my install of my 536 and FT-8800. Some say that if the antennas are to close together the power when TX'ing on the 7900 may damage the scanner. Mine are 28" apart. I have never TX'ed on the HAM radio while the 536 was on. I have TX'ed with the 536 off and so far no issues that I know of.
Just an FYI. I do not want you to damage your scanner.
It doesn't quite work that way.
Turning the power off on your scanner does not disconnect the antenna. Even with power removed, the antenna is still connected to the soft bits inside the radio that get damaged when too much energy is introduced into the radio by closely spaced antennas.
If your scanner has survived, then they are sufficiently spaced.
Keeping antennas properly separated is a good idea, though. Coupling between antennas, in this case RF energy from the FT-8800 transmitting, getting into the scanner antenna, and eventually making it's way into the front end of the scanner, can be problematic. There are a lot of variables, though:
Transmit RF power
Coaxial losses in the transmit antenna line
Coupling between the two antennas (distance)
Coaxial losses in the receive antenna line
Type of protection (if any) in the scanner.
Better safe than sorry.
You do want to provide a sufficient ground plane under the antennas. For the dual band mobile, you want (ideally) 19 inches of metal all the way around the antenna. This will give it a nice radiation pattern and will provide lowest reflected power.
The scanner antenna isn't quite as critical, but it'll perform better with a decent ground plane.