Hm-m-m-m, good question. True, the parts book says it takes two, but shows only one in the pictorial. However, that in itself is not unusual. Note that it says it takes four screws, but only shows one in the pictorial. That is common practice throughout the whole OMC system.
So, the question becomes how many gaskets does it really take? A photo of the contents of the water pump kit shows only one gasket. So it would seem that is enough. But there really is no reason you can’t add another below the plate if you so desire. Gasket or not, it is important that the plate be sealed to the bearing housing to prevent sucking air.
