It’s called double particles – they don’t combine, they’re just performing separate functions. The に is marking 私たち as (I think) the indirect object of the sentence, while the は is marking it as the topic. To be honest, I’m not entirely sure what the に is doing in that sentence, but to give another example:

ここでは友達と会います。

This basically translates as “Here is where I am meeting my friend.” The で is still marking ここ as the location, while the は also marks it as the subject.