Can USB and MIDI be used simultaneously?

No you cannot use the MIDI direct from the keyboard when connected via USB.

The MIDI OUT from the keyboard will now become a MIDI OUT from the PC so if you use a capable DAW (Logic should be capable enough) you can usually setup some MIDI routing to send from your track input to an external output so you can connect to external gear.

In Ableton Live there is an "External Instrument" device which is added to a track that takes the MIDI input and simply sends it out again and can then optionally capture audio back from your external device making it a part of your project. That is how you would have to work, you just need to find out how to do that in Logic.