I think using two different backends for the same app might cause some friction as you might have to transfer some data from to another.
Personally I’m begining to like Backendless for some of its features especially the ability to host my own instance.
So will consider it for some future projects where I require less server-side application logic.
But coming back to your use case, if the only reason you want to use backendless is the stripe integration then I will advice you not to use both.
You can do the same stripe integration that Mark showed with backendless in bubble easily.
I’ve intentionally refrained from showing how to do it in Bubble because while this approach is easy to integrate its going to take a lot of effort to be PCI compliant.
You can read the discussion I had with Mark on the post he showed the video of the Stripe integration in Backendless.
Also in terms of cost note that both bubble and backendless are not free so using both means paying for two backends.
So I will advice you to decide on one and use that exclusively.
There are many good reasons to use Backendless to power your dropsource app but if the only reason you’re considering is the stripe integration demonstrated then I will say don’t.
Because you can do the same stripe integration with Bubble.