There's still a lot of variables, like antenna gain, feedline losses, etc.
Based on an 80 watt radio, 6dB gain base antenna, I'd start looking at 20 feet or so, just to be safe.
It would be beneficial to have one of the antennas physically higher than the other, to get that horizontal separation that
@vagrant touched on above.
That should help with reducing the chances of actual damage to the scanner.
It likely isn't enough to prevent the transmitter from desensitizing the scanner. It's likely that when you key up on the 2 meter radio, the scanner will lose reception.
Those RF limiters that vagrant linked to above would be a good thing to install at your scanner.
I've got a remotely controlled receiver at one of my tower sites at work. There's two 100 watt VHF repeaters there, as well as 3 cellular carriers and a 100 watt 800MHz repeater. When our PD or Fire repeater kicks on, it'll wipe out the RX on the receiver. That's with a 0dB discone antenna on the receiver, and about 6dB of antenna gain on the repeaters. There's about 30 feet of horizontal separation between the remote receiver and one of the repeater antennas, and about 50 feet on the others. No damage to the receiver, but it does suffer from that desense when they transmit.
