Using the Intercompany module.

Inter Entity Transactions and AR-AP Settlements if needed. We use these products for doing branch transfers primarily.

Small society = small budget?

DIY small, yes super small.

Sage's Intercompany Transactions module does this perfectly.

Intercompany Transactions will do the AP side and intercompany GL entries, but not AR.
But ICT is an add-on that you will have to pay for, it is not included unless you bought a bundle that has it.

Why not create all the entities in the one database and have the identifier in the account code for the entity eg co-accno-division then you can simply do your transactions across entities as needs be but just control it so that your trial balances don't become unbalanced. The Orchid Inter-Entity module is brilliant for that. But even without that module, you simply have all the transactions in the one database. You can then do bank transfers between the bank accounts and it can be quite simple.