Q about Time Machine backing up to two disks

Inside my MB Air Time Machine Prefs, every time I switch the disk I am using for backup, I get a prompt asking if I want to use both disks.

I use a Synology NAS connecting over wi-fi as well as a 3TB hard drive which I connect by USB when the MB Air is sitting on the desk charging. I have always chosen to not use both, but the other day after I did my backup to the USB drive, I neglected to switch back to the NAS. As a result, TM did no backups for 3 days...oops.

The USB disk is not always mounted. Most of the time the MB Air is not sitting on a desktop, it's in my lap. If I choose to let TM rotate between the disks, will it throw a hissy fit if the USB drive isn't mounted or will it just go ahead and keep backing up to the NAS every hour until it sees the USB disk mounted again.