It depends which service you want to have on each freedombox.
For what is accessible with http(s), 2 Freedombox setup -- cannot conect to XMPP server on Freedombox 1 - #2 by Avron explains how I used to do it.
However, for anything else than http(s), I don’t know. I can ask people who I know run a number of services while they have a single public IPv4 address, but I expect that this may involve some complexity, and require running software not handled by freedombox.