One way this could work is -- with support from your app that interacts with the federations -- the following:

Suppose you have 100 fmBTC in Federation 1 (we'll denote as F1), and your current balance in federation 2 is 0 or very low.

Now imagine that you wish to pay someone in federation 2 50 fmBTC eCash. Clearly you don't have enough balance in this federation, but you do in F1.

A smart wallet could at the time you attempt to pay your friend in F2 (where you have zero balance) perform a multi-mint swap transparently in the background: it would mint a 50 sat eCash token on F1, direct a lightning payment to the F2 gateway with your own F2 account as designated beneficiary, which would materialize a 50 sat eCash token in your F2 account, that can then be sent to your friend.

Although this capability is not present in the Fedi alpha app (AFAICT?), it is present in the eNuts client for Cash (a similar, but non-federated Chaumian eCash system)