One of the 12AT7s is the reverb driver. The tube has lower gain than the 12AX7, but it also can handle a lot more plate dissipation. The reverb driver is in fact a tiny power amp driving the reverb pan as though it were a little speaker. It really wants to be the proper tube.
The other 12AT7 is your phase inverter. I recommend that stay a 12AT7 as well.