Actually 6 dB may be of some help, since the problem is probably caused
by 3rd order intermodulation between two signals and the attenuation of
the intermodulation product will be greater. However, as the others
have pointed out a low-pass filter is not likely a good solution.
Either the notch filter suggested or a bandpass filter should be
better. There are some 2m bandpass filter designs in the RSGB VHF/UHF
Handbook 3rd edition, if you can borrow a copy.

73,
Steve VE3SMA