Well, normally I'd say don't do it.
With a passive crossover network, the crossover frequency is dependent upon the impedance load connected to it. So, if you have a crossover designed for a 4ohm load on the mid output, putting either a 2ohm or 8ohm load will change the crossover point and other bad stuff.....so it's usually not a good idea.
But, ID makes 2ohm mids, which means they likewise have to make crossover that is compatable with a 2ohm load. Whether they designed a single crossover to handle both loads, or use two different crossovers (one for 4ohm mids, one for the 2ohm mids) I am not sure. That's something you would want to check with ID. If they do it with a single crossover, it shouldn't pose much of a problem for you to do what you want (as long as you wire the mids in parallel). If they use separate crossovers, then it's just a matter of you purchasing the crossover designed for use with a 2ohm load.
Okay....and after looking over ID's website, it looks like they use separate crossovers; one called the "X2" for use with 2ohm mids, one called the "X4" for use with the 4ohm mids. So, it should be as simple as you purchasing the "X2" crossover and wiring your two 4ohm mids in parallel for a final 2ohm load.
It might still not be a bad idea to check with ID (or one the ID forums) before you do it......but I see no reason why it wouldn't work (other than maybe ramifications of poorly setting up the dual mid setup).
Where are you locating the 2nd mid on each side? And why are you going with a 2nd mid rather than just like upgrading to a 6.5 set?
