I did some work about 7 years ago, where I needed two serial devices to both feed a computer. I found the following:

[Link: infogrip.com]

It is designed for serial mice. I found it worked nicely for my serial application. You can email the company that designed it, and find out if it fits your application. They might need details on the handshaking requirements of your devices, baud rates, data length and parity. They were very responsive years ago when I was using their product.

Good Luck!