Be aware that no gtr preamp is stereo! The fact it hs 2 outputs doesn't mean it's stereo! It simply has a splitted mono output.
There is no way one could use a stereo FX processor IN STEREO, with a mono head!
You should use Eclipse in mono. The best way is to connect the head FX loop send to Eclipse input 1 OR 2 and set Eclipse input mode to 1 or 2 accordingly. This is available under the LEVELS key.
Then both Eclipse outputs should be connected to 2 channels of a mixer and pan them full right or full wet. Then use only the left OR right output of the mixer (depending on where you panned both channels to) to the head fx loop return.
That'll make the best compatibility.
Of course you'll need to use the Eclipse XLRs I/O as these technotes explain:
all the best