You cant, as such. If you want to use the preamps on BOTH amps you'll need to have stereo In and OUT on the pedals.
Otherwise you have to use the preamp of just one amp. This pedals are connected to the send/return of this amp as normal. This amp is operated as normal.
The final stereo output of the final pedal goes to the effects return of the other amp. Thus this amp is not using the preamps (distortion etc.) and is only using the power amp section (volume).
I just run the amps clean and use a good quality overdrive as my preamp with the CH-1 at the end going stereo... i'm not sure that using two chorus pedals would give you the true stereo effect,what i've noticed about the CH-1 is it doesn't run a 'dry' output but seems to modulate it's effect between the two when used in stereo....i use a CE-2 and a CH-1 on my board and the two on together with the CH-1 running stereo at the end in a great sound....
Dirk and Paul H's suggestion would work if both amps are the same.....otherwise you would have to go with one preamp as pointed out above.....
i use a two different amps for the mixture of tone they give...a 70's marshall and a musicman combo so if you also use two different amps this might be something to consider..i.e....losing the preamp tone of one of the amps affecting your overall tone/sound......