The reason you are not able to use your USB Mic and Abox iTwo at the same time is two fold.
1) Your computer sees both the iTwo and the USB mic as independent audio interfaces and needs you to make a choice of which one to use. It can only use one audio interface at a time.
2) Many USB Mics work in tandem with Windows Audio and you would want to consult your USB Mic's Manual for proper setup not only on your computer, but also with use in a DAW.
Not sure 100% how you are setting up your iTwo, but keep this in mind....
-The iTwo is an upgrade to your computer's on-board audio card. It replaces your computer's audio card in the signal chain. So, any audio you are inputing and or outputting, when using the iTwo; will be going in and out of the iTwo. Speakers and or headphones will be necessary and directly connected to the iTwo for proper usage.
This brochure has two hook up diagrams on it, showing how it can ben connected to a computer:
As far as latency, all interfaces will have some latency. Latency is the round trip time it takes for a signal to be deployed, travel into the interface, into the computer, through the software and then back out via headphones or speakers; in this case. No matter if you use the USB mic or the iTwo..you will have some latency. This is where proper setup of your sampling rate and block size play a big part in your overall digital audio recording experience.
If you are using Synths, VST..etc; then you would need to stick to the iTwo and see about adding some non-USB Mics to your arsenal.