This time I tested using two external flashes and I really like the lighting, even though the only picture I got with both flashes working was a photo of myself (second photo below), after which the secondary flash died. The problem with the secondary flash is that I couldn't get it to go into sleep mode with my current set-up, so its batteries drained pretty fast unfortunately (a matter of days). So after this I would continue with a single flash.
The set-up with two flashes (primary flash to the right, secondary flash to the left) (2017-04-20)
Yours truly testing the set-up with two flashes. The blue suitcase is what I use to carry all my camera trap gear (2017-04-20, 17:59:38)
A mountain hare (and some condensation on the lens) (2017-04-27, 01:23:56)
A mountain hare (and some condensation on the lens) (2017-04-27, 01:24:20)