Launch early and do both. Then you are going to get it wrong, so iterate and focus on figuring out mutual requirements, pain-points, and integration points.
Doing one or the other first just means you delay the moment where you find out how wrong you got it. Build a prototype UI with a functioning backend as early as you can. It doesn't have to be pretty but it does need to be functional.
So, figure out some MVP with both frontend and backend covered that is demoable/usable enough to plan further steps.
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