On the "better" of the two twin mowers, put in brandy-new spark plug (w/ spritzDoes it "try" to kick over?
Anyway, if you were not able to solve it with simple stuff like that, the next step is to either get good and small engine diagnostics/repair (AKA DIY) or to take it in.....
of starter fluid), put in brandy-new battery. Allowed to charge for an hour or
so. And, it DID start.
degree temps helped a bit.
I added a bit of stabilizer and gave the mower a good shake. I will start it up again
on Thursday to let some of the treated gas get into the line (which is more than I
have EVER done in the past). Too much in the tank to just let it run dry. And
remove the battery.

NCIS comes on afterwards
but there's no doubt! 