This is not supported on our switches today, but feedback like this is exactly what we need to prioritize what features we work on. If you haven't already, make sure to use the "Make A Wish" box on the bottom right-hand corner of every page on Dashboard.
That is exciting and terrifying at the same time. We gave up this feature 2+ years ago when we switched to Meraki. We were a bit surprised it was not available because we had run it for years on our Procurve switches without issue. Even without multi auth, the 802.1x stack in the Meraki switches was pure garbage until we got on the 9.xx firmwares. Multi auth really helps us be flexible with our ports. We had to run several new home runs when we switched due to the single authenticated device limit.
Any future plans to support guest vlan with multi auth? It has been a few years since removing our last procurve but as I recall, guest vlan still worked if the only client on a port was unauthenticated. If an authenticated device was already on the port, then the unauth device was blocked.