If the forward gain of a six-element Yagi is about 10 dBi, what would the gain of two of these antennas be if they were "stacked"?

When you stack two identical antennas and feed them in phase, you are effectively doubling your radiated power in the primary direction. In the decibel scale, a doubling of power corresponds to an increase of exactly 3 dB. Therefore, if one antenna has a gain of 10 dBi, adding a second identical antenna theoretically adds 3 dB to the system, resulting in a total gain of 13 dBi. Decibels represent a logarithmic ratio, so you cannot simply add the original gain values linearly.