Currently got Stan’s in my tubes but picked up some Bontrager fluid yesterday.

Is it OK to add the Bontrager stuff or should I clean the tyres out and start again?

Given they are completely different “types” of sealant (ie. latex based and non-latex based), I’d be tempted to start afresh.

Though maybe you’ll get the best of both worlds – allegedly better sealing from the Stans and much less prone to dry out from the Bonty!