I think there is value in what you suggest, depending on the communities. I accidentally created a situation which I think is pretty similar to what you are talking about.
We have a symbiotic relationship with the Discourse meta community. We have quite a few members in common, which makes sense given that most people using Discourse are trying to build a community of some sort. It’s valuable for us because it is a growth channel and it is valuable for Meta because they can keep their concept focussed by directing more general community building discussions over here.
I didn’t plan it, so I can’t say that I used specific tactics, but it formed organically after I built relationships with Jeff and @erlend_sh.
If I were to do the same thing actively with another community I’d start by building a relationship with the CM. I think the reason that it works well here is because we have mutual respect – I don’t spam meta with links but when something comes up, Erlend sends them over.