[larry.said]

The funnest thing to do would be to build a Y splitter yourself with a rotary switch which will equalize the input impedance of the two headphones. So lets say your using K701s with SR225, you'd put a 32ohm resistor at the input of the grados so that both will be fed the same current (they are at the same potential being in parallel). Of course this isn't gonna work perfectly, but its pretty cool.
You'd have to use a control though: i.e. you'd always use K701s and change the other headphones, or else it becomes too complicated.

 

[Trance1]

Hmmm, going with a y-splitter rather than focusing on a two jack amp does make the most sense to me.

Thanks for all the informative input! Much obliged.