Transparently bridge network over serial link

I have two Linux devices - for simplicity let's call them Raspberry Pi's - in a box that I want to connect to a network with a singe Ethernet link. Both of them need to be accessible, individually, on the network. I can join one to the other through a serial port very easily. It looks like pppd or slip may be the answer, however all the examples I've seen so far use fixed configurations which is no use to me - **both** boards need to be able to take on **any** IP address either via DHCP or manually configured, independent of configuration on the other. This feels like it should be fairly easy to accomplish with Linux but either I'm not finding the right info or I'm using the wrong terms while searching. Any thoughts appreciated.