It sounds like you're trying to avoid the latency of waiting for a write confirmation to a remote data center?
App ==> DC1 ==high-latency==> DC2
If you need the write to be confirmed before you consider the write successful in your application (definitely recommended unless you're ok with losing data and the app having no idea), you're not going to solve the fundamental physics problem of having to wait for a round-trip between _something_ and DC2. DC1 can't acknowledge the write until it's in memtables and commitlog of a node that owns that data, so under the hoods it's doing basically the same thing your app would have to do. In fact, putting DC1 in the middle just introduces a (possibly trivial but definitely not zero) amount of additional latency over:
App ==high-latency==> DC2
The only exception would be if you had an expectation that latency between DC1 and DC2 would be lower than latency between App and DC2, which I admit is not impossible.