• There's federation, which is when the instances discover each other. It can be inspected at /instances/. The instances are already successfully federated.
• There's community discovery, this happens when a user searches a community.
• There's post/comment/vote replication. It happens when a user subscribes to a community.

The reason these these happen lazily, when a user requests (via search to discover, or via subscription to replicate) is so a small instance doesn't have to eagerly replicate the entire fediverse worth of posts that no one will read. Things only get copied once someone wants them, which is a general principle that is important to keeping the federated network healthy.

Now, there are also usability concerns... of which I regard community discovery as the most serious. But I'm sure the devs will be thinking about all kinds of ways to improve usability, they will have to balance federation traffic so small instances remain viable and there aren't millions of federation messages flying around to ship data no one wants.

I guess a way to force each server to federate with eachother would be to use accounts on both and cross search on each one?

I would rephrase it to say... the way for a community to be discovered or replicated to a remote server is to ensure there's a user there who is interested in it. If there is, they'll search and subscribe. If no one on the server is interested in the community, yeah, it won't be discovered or replicate.