Can I have two boots from same Pi 400?

I'm not well versed in technical jargon.
I have Pi 400 all set up under it's own internal O/S card/drive.
The Pi boots from it's own internals, not from a USB thumb drive.
Is it possible to install Ubu 20.04 on a USB thumb drive and boot from that into Ubuntu or remove the USB and boot from the default internet card?