Absolutely! I've built a board that converts one serial port to 8. There are two main "catches". The first is that a picaxe can't listen to two ports at the same time, so you need a protocol with some sort of handshaking. The easiest is to use the RS232 CTS and RTS lines - put a line high to say "I have some data" or "I want to send some data". The second catch is that the Serin will hang a picaxe, so you might be waiting for data forever on one port while data on the other port is being missd. So again, you need some sort of protocol. There are other ways as well - eg one picaxe just sends data in packets every second, and then the receiving picaxe will only hang for a second.