OK…
Try This.
Instead of using sends (switch them off) put your amp sim into an insert of a group channel, then set the instrument channel outputs to the new group channel. All the signal will now be sent to the FX without any bleed from the original channels and you can also use the channel panning to separate the sounds (if the FX is a true stereo plugin)
Edit.
Ah… two amp sims.
That will only work with one amp sim, there is a way to do similar with two.
In fact if you use your original setup but set the sends to pre fader then turn the channel faders down all the way that would work but without the panning, I don’t think your version of cubase supports send panning.
If you want the panning option you would have to use my first setup, but create another group channel and use the second send with its routing set to the input of the second group.