Using two headsets on my MacBook Air
You need a Y cable splitter to connect 2 headsets.

Most come with single audio control but there are some with dual audio volume. If you already have audio volume control on each headset then no problem.

Since you have the headsets already with different plugs you will have to get appropriate cable splitter for that.

I did some search for you.

First the USB male to 2x Female splitter like this.

One you use for your USB Head set.

For the second headset you need another adapter like this.

Those do not have Volume adjustment, so hope there is no problem with that :).