Just said I'd heard it worked. Never said I'd seen it done or done it myself.
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Quando Omni Flunkus Moritati
The PT2 heads have the same base and threading as the PT3 dual heads, so they will screw into the base. However you may get some leaking by the center post. The rubber grommet is to prevent leakage from the tank to the airhole. On properly fit units, the center air shaft will mate with the top of the head and the gasket will prevent leakage.
When you put on the smaller head, the center air shaft won't fully move the gasket down. As a result, the rubber grommet will still be resting somewhere up the chimney on the shorter single head coil.
Depending on your juice, there may be sufficient clearance for liquid to seep into the air chamber via the grommet since the grommet isn't sitting flush with the top of the head.
You could always lift the rubber gasket up onto the chimney so that the airshaft pushes the gasket down the chimney as you screw base onto the tank. This won't create a full seal, but should seal it enough to "minimize" any serious leaking, although some is always possible.