Are both Slingboxes on the same network? The answer may be no, but possibly circumventible.
Recall the slingbox communicates with the internet via your firewall (a single public ip address) thru port forwarding defaulted to 5001 on older models. Your firewall must choose which internal private ip address to send the 5001 data stream. Since the slingboxes by default must compete for that single port stream, I can imagine only one can be getting the connection at a time. However, on older models (not 350/500) you can change the listening port, so one box could be kept on 5001 and the other can be whatever you like. I have not tried this myself, but it should work.
I cannot imagine a work around for the new boxes. Slingmedia should comment on that.