DELL UPS 1000T: simultaneous control connections?

I've got a DELL 1000T TOWER UPS which I want to hook up to my PowerEdge T420 AND another NAS I have on site.

I was thinking of using the serial port to hook it up to the T420, and the USB to connect it to the NAS. That latter part works fine, but the RS232 seems to lose connection on and off.

So, would this be a faulty cable, or is simultaneous use of both USB and serial port not supported by the UPS or something?