The answer is "both of them". Pick a client that makes it the easiest to connect to separate instances on separate accounts, then choose the same-ish username.
Federation is, fundamentally, an anti-feature that exists solely to create liabilities for the end user as, by design, it empowers people who should not be empowered to destroy your social graph arbitrarily (the "not sharing my ban list is bannable" problem plagues the Fediverse).
This is not something the people writing the documentation understand (or worse, the above is a feature for them), but the correct way to use the service is to engage atomically under multiple different identities rather than be caught in the fallout of petty admin slapfights, just like signing up for a bunch of different phpBB forums was before the rise of Digg/Reddit.