Onyx 400F - a stupid question about outputs

Hello all,
 
I am using an Onyx 400f but would like to add a second pair of powered monitors to the setup.
 
The Onyx only has 1 pair of dedicated monitor outputs on the back - can I use a spare set of line outs to feed the 2nd pair of monitors, or will that cause bad things to happen?
 
If it's a no go, can anyone suggest a reasonable solution which isn't physically huge or too expensive?
 
Thanks in advance! 
 

Thanks Mike - I just didn't want to blow anything up. 
 
The onyx has a kinda virtual mixer thingy which should allow me to switch the other monitors in quite easily, while using the knob on the front of the onyx rack itself to turn down the other pair.