TCP/IP config question - using one i5 as a "gateway"/"router" for another?

I have two i5s, physically located next to one another in the same room. They both need to communicate with printers, PCs etc. physically located elsewhere in the building.

One of the i5s only has the 2 embedded ethernet ports and no other ethernet port. The other i5 has a third ethernet port. The first ethernet port on each system is reserved for the LAN console. At present the second ethernet port is used to connect each system separately to the LAN so they can communicate with the PCs, printers etc.

My problem is that the LAN generally is slow (for reasons I won't go into) so sending a large files between the two i5s can take quite a while. I've tried linking the two i5s directly so the data doesn't have to flow around the slow LAN just to get from the one machine to the other and file transfers are dramatically quicker. The problem is that then means the one i5 is not connected directly to the wider network and cannot communicate with the rest of the LAN.

Question: is it possible to configure things so the i5 B physically connected just to i5 A, and not to the rest of the LAN, and have i5 B communicate with the rest of the LAN through i5 A, using it as a gateway or router?

I'm guessing this should be possible but haven't worked out the TCP/IP interface and route settings to do it.

Can anyone help?

Pete