Huh? If the preamp and headphone amp are seperate circuits, ok, but any amp is going to behave differently when you plug multiple things into it.
For multiple headphones, it really depends on what headphones you are talking about. If you plug in two K340s, the impedance gets halved and in some cases they might even be easier to drive this way in terms of voltage. If you plug in two Grados, you are talking 16ohms... pretty low for some amps. Two headphones of different impedance are going to make volume level matching pretty tough.
Now, he is driving a pair of speaker amps and a hard to drive headphone at the same time... I think it goes without saying that this will put a lot of strain on the amp. It is not surprising that the headphones sound off in this situation.