You've got loads of ways you could implement this.
From your post, I'm assuming you're not going to use RMI.
If so, then it might be worth creating a little protocol for your system to use. For example, the first word use in the communication is the operation required, following by its details. For example:
The first command asks for the balance of the branch that the command is sent to. The second command requests the transfer £321.55 *to* the branch that the command is sent to. The third command requests the transfer of £21.32 *from* the branch tha the command is sent to.