I hope you are forthright and honest with your clients and disclose you are using a single-card camera that is inferior to real “professional” level equipment...
No kidding. I remember how many times film photographers would notify their clients in advance how their cameras only had one film slot and there was a 50% chance of losing the images because there wasn't a second one copy of their wedding photos.
Lots of things can happen, you can be walking to your car and get knocked over the head and lose all your dual-card-slot bodies and cards.
Seriously, I really do want dual card slots and I even refused to buy the EOS R at first because of its single card slot, until I actually got one in my hands. Happily using the EOS R for now and plan to upgrade as soon as Canon finishes building my next camera!
Just because the EOS R isn't a professional grade body doesn't mean you can't get professional results from it.