err... best NOT to connect headphone output to the input of another amp.
again, HarrisonYeo, what is the purpose/effect you are doing this for?
if you have a multi-efx with stereo output (left-right), you can simply connect this to each amp. just create a patch that has output to only Left and another patch with output to only RIGHT. This way, you can drive whiche amp you want. to drive both amps, just create another patch to output to both (LR).
loads of experiments you can try. with 2 amps, you can do S-T-E-R-E-O !