Nope. The only concern with multiple Slingbox on the same local network is bandwidth. If you're using at least a Fast Eternet (100 Mbps) hub then you'll be fine. Hardwired is better than wireless due to protocol latency associated with wireless. A switch is better than a hub as it doesn't share bandwith like hub. Gigabit (1000Mbpx) Ethernet is obvioulsy faster than Fast (100Mbps) Ethernet.
The only other caveat would be remote viewing the seperate Slingboxes. That's when you'll need to make sure that each Slingbox is using a unique port number. That's because your firewall can only port forward each port to a single IP address. So, if remote viewing is important to you for both boxes then you'll need to change the default port from at least one Slingbox, and create a matching port fowarding rule for its new port.