As Mel mentioned, almost all pitots have drain holes. The holes are part of the design, and the pitot is calibrated for these holes. If you work out the fluid dynamics, you'll come to see that as long as your drain holes are much smaller than your main pitot opening, the drain holes don't change the pressure output of the pitot at all, since the flow through the tube is minimal and thus the frictional loss is also minimal.
The drain holes in a Dynon AoA pitot are much smaller than the ones you see in the back. The ones in the back are actually drains for the cavity inside the pitot mount, while the actual pitot pressure lines have much smaller holes that vent into this internal cavity.
The ASI in the Dynon EFIS units is calibrated just like every other ASI on the planet is. A certian pressure equals a certian airspeed. We use the same tables as everyone else, and we can use any standard pitot for airspeed measurements, and our AoA pitot acts just like any other pitot you can buy.
So to directly answer your question, you can use the Dynon AoA pitot to drive any ASI, including the Dynon EFIS and a mechanical gauge at the same time. We are calibrated the same as all other pitots. The gentleman in the forum that you mention is mistaken and does not need to have two pitot probes on his aircraft.
