Crosstalk in analog switch
It's normal to be so. Here is the equivalent resistance circuit @1MHz:



When ch1 is ON and ch2 OFF, the 75\$\Omega\$ is shorting C8, thus the signal comes unaltered through ch1, while through ch2 it first goes through C4, whose reactance @1MHz is ~319k\$\Omega\$, thus forms a divider with the 10k\$\Omega\$ grounded at ch2 input.

When ch1 is OFF and ch2 ON, C8 will conduct, mostly, with a reactance of ~159k\$\Omega\$ (in parallel with 1M\$\Omega\$), and the signal is attenuated first through C8 and C7||R4, then attenuated even more through the divisor formed by C5 and C2||R2,while on the other branch, it\'s only attenuated through C4 and R5, then going clean through the 75\$\Omega\$ of ch2 ON.

As you can see in the picture, when ch1 is ON it's ~21.5mV, while the other is ~10mV.