Frontend/backend team with different technology stacks

Hi all,
I would like to know your opinion on a team setup for three frontend/UX
developers and a bunch of backend developers - whether putting them
together in a cross-functional team makes sense in a particular setup.


My understanding is that the benefits of a cross-functional feature team
lie in helping people better deliver customer value by
- team helping each other to get things done
- keeping focus on the feature with common backlog and low total WIP
- reducing hand-offs
- etc.


Now the problem is that we have no full stack developers after a recent
reorg; frontendists have JavaScript-based stack, backendists Java-based
stack with little shared knowledge and technology. It seems to me that the
above points are not satisfied:
- the two groups cannot help each so much due to little shared knowledge
and unidirectional dependencies
- the two groups frequently get out of sync because, e.g., designs for new
features must be prepared in advance; this leads to high total WIP
- because of the out-of-sync problem and unidirectional dependencies, there
are still hand-offs, just hidden inside the team


Separating the teams smells with waterfall, but does joining the two groups
together in a team outweigh the cost in this case?
Does it make sense to split the frontend team and perhaps make the backend
part more cross-functional towards the core of the system instead?
Does anyone have successful experience with frontend+backend teams without
full-stack developers (or teams with little shared knowledge in general)?


Thanks,
Jan