Can a UPS have dual power inputs?

So I did a very little bit of googling on this and it seems up in the air.

For complete redundancy, it would make sense to have a UPS powered by two separate circuits if possible, though I've never actually seen this done.  But it appears that this may be some sort of code violation and other posts suggested ( in much more technical detail than I understand about power supplies ) that the UPS failover system could not work if it had two inputs.  

So, question is, have you ever seen this done and do you recommend it?

What we have now is two UPS each fed by a dedicated circuit and dual-power supply servers plugged into each UPS.

Thoughts?