I spent nearly 40 years as a RF communications technician and have seen this sort of problem before. You need to get at least five or more feet between those antennas if you are going to run 75 watts at VHF frequencies. The scanner front end circuitry won't be able to withstand more than a few tens of milliwatts of incident power being applied before bad things happen.
If you are unable to physically separate your antennas, then you should contact somebody like PAR electronics about a custom notch filter for the antenna of your scanner tuned to reject the APRS frequencies you are using. If you can get around a 30 dB notch, that should be adequate, especially if you can get a little more separation between your antennas.
73....John
W8JNC
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