I really haven't actually seen an active pedal that does this by nature
because the results are usually undesired (you'll see and judge for
yourself).  Summing a stereo effect might give you undesired effects through phase cancellation.  I highly recommend you simply unwire the stereo patches of your board.

but if you insist…  a passive box like the Morley ABY will get you results you're after wiring it backwards.  there may be impedance issues as each output tries to drive the very low impedance of the other, forcing
both outputs into current-limit . As a minimum,
severe signal loss may result.

the Toadworks Ajax is an active pedal that will do the job better.  even with the phase switch results will vary.

let us know how it turns out!