On this occassion I trusted the split between two bodies. I tried to do instant backup to my phone but I can't seem to get it to work. Ideally I would always do that as the phone is always in my pocket.
It's just getting it to do it. I'm currently considering getting a second R as 75% of all my photos are now taken with it and would like to split the load better.
If you're in my shoes partly .. possible wait for the pro or version ii of the EOS R to arrive? I'm keeping my first EOS R and will buy the next version with two slots. I have 1/3 of the estimated cost set aside already.
I haven't done a wedding yet, but have a plan for my EOS R. SD cards are inexpensive and failures very rare. I would swap out cards about every 16 MB worth of shooting. So instead of everything on one big card or two, it would be split onto 4 cards. With some on my second 5DS body and it's two slots.
So far, I've been able to get the WiFi transfer of JPGs working but only tried it for a dozen. When I have a man or woman over for a model test shoot I may try a full set for a test.