They would be huge.
I doubt you could use two together in the back on the tandems without them rubbing each other or the tandem drive housing. You certainly want all 4 tires on the rear the same size because there is no differential between sides or between front and back on each side.
Tires for the 212 are a bit scarce. Seems like several years ago Denman made a commercial tread (or maybe it was a spade grip/farm tread) for that size.
Miller sell various x24 sizes (they list an 11.5x24 with 10x24 in parentheses I guess as a compatible. Take a look at the various options on this page http://www.millertire.com/categories/rear-tractor-tires/
I get my lawn tractor rubber from these guys and have been happy with them. They have a bunch of less common stuff as well.
I have 11.2 x 24 tires on our machine. They work well and there are no clearance issues. These were new old stock Goodyear with more than adequate load rating for the machine.