Line 6 expression pedal inputs

[Finbar]

Very specific question!

I've just bought two M5s off pilotbicycles, and I'll be putting one in front of my amp and one in my FX loop. I used to use an M9 many moons ago, so I know most of the ins and outs of the unit. 

However, I'm wondering if it's possible to use one expression pedal to control parameters on both units? When I was using the M9, I could control up to 3 effects with one expression pedal which was pretty gnarly, but with it split across two units, I don't want a second expression pedal on my board if I can avoid it. There's no requirement for TRS cables with the Line 6 stuff,  it just needs a mono signal. So if there was a passive splitter between the pedal and the two units, I assume this might just work? 

Not using expression pedals wouldn't be a dealbreaker, but does anyone have two M units and an expression pedal and could test it out? Or just a sensible reason for why it would or wouldn't work?

Edited March 29, 2019 by Finbar