Daisy Chain DE2 boards

The simplest way, is just cut the TX line from the second slave, and have a protocol that doesn't require acks or responses. But you can use an RS232-RS484 converter, and then address each slave to respond. (With a serial protocol)

The issue is your max speed at RS232 is going to be around 1 Mbps. and it's serial, so you divide the number by 10 to get your throughput. It's fine if all you are doing is sending commands, but if you are trying to push a bunch of data between the boards, I think you'll be stuck.

You can find RS232 to RS485/422 converts at lots of places, including Amazon.com.. (just do a quick search). It's basically similar to RS232, but is meant as a multi-drop system. So you can have more than two devices on the bus. RS485 vs RS422 is just Half Duplex-vs-Full Duplex.