From the poking around I have done, It seems that 22" x 14" x 9" is a relatively universal standard for official carry-on size allowance, but that for personal items there is substantially more variation mostly ranging 17"-18.5" x 10"-14" x 7"-9".
Are personal item sizes frequently enforced?
Would it be relatively safe to make it to the gate with two carry-on size bags, one spinner and one duffel-style sitting atop the spinner?
Or would there be a reasonable risk that one of them would need to be checked at the counter?
Practically, the difference is whether I need to look specifically for an "underseater" spinner or if I can look for a regular carry-on spinner to go with my 22" x 12.5" x 9"
suiter duffel.
For context, the primary concern is for domestic travel where time is of the essence and checking a bag at the counter is highly undesirable because of the wait to collect it at the destination...