You can have more than one Slingbox on the same network. I've got two hooked up myself.
The trick is making sure each one has it's own port. Have a look at this thread:
To make the changes you need, look for manual network settings in the Remote Viewing section of the Setup Assistant. That's where you'll find the place to give each Slingbox its own port.
In order to make both Slingboxes watchable from outside your home network, you'll probably have to make a simple addition to your router's port forwarding rules.
If you need more help, post back and ask. It'll help if you say what brand/model router you have.
Hope this helps,