Can I wire 2 HD antennas together?

you would need a combiner which would introduce a lot of loss so it is not a good idea if you are in a weak signal area. combining DTV signals actually works better than combining analog TV signals, that is because DTV has more multi-path rejection capability. multi-path is the problem when combining antennas like this, that is because the "unwanted" antenna still receives some on-channel signal via back scatter reflections. the only time combining antennas works as a practical matter is when one is on UHF only and the other VHF channels only and the combiner can reject the unwanted bands, or using antennas that respond to only one television channel and rejects adjacent channels. most consumer TV antennas are designed to be broadband and receive all channels.

Probably not with a single combined feed line. If you did some kind of A/B/C switch or multi-input phase control unit you might be able to make it work. You'd probably be far better off getting an antenna with directional gain and using a small rotor to redirect it. Otherwise, if you have close stations, try a simple omnidirectional antenna and see how that goes.