Probably they are both the same unit, but maybe one is newer than the other. The 11 refers to the length of the rod in inches, and 15 is the current rating in amps.
I can't think of any reason why there would be two wired in series, but surely both thermostat rods can't fit in the same thermowell, can they? Or has one stat had its rod removed?
They don't appear to have a visible a 'cut-out' button on them either, so I would assume that this is the type of thermostat that should be used with a separate 'cut-out' something like this:
View attachment 168603
Is it possible that the second thermostat has had its rod removed and then been wired in where the reset button should be?