sure it is ... one thing that u could do with a setup like that is build a ported box for the 12's tuned higher to get more spl out of them and have the 15 pick up the lower frequencies.. u could have them all pickup the same frequencies, it might sound a bit off though
Basically the music would sound 'colored' & un refined . It'll be like they are competing against each other & to set the gains & freq would be a nightmare . How would your car ride if u had diameter wheels .
If u really want more loud throw in another type r 12 or get rid of the 2 type r & buy 2 type X
2 type r = 1000wrms , 3 type r = 1500wrms OR 2 type x = 2000wrms !
Really, the only way to mix sizes reasonably would to have those 2 12's and say an 8. There wouldn't be AS MUCH cancellation. But 12's and 15's are normally relatively close in the frequencies they hit.
I actually just installed my two 12" type X's today with the alpine specks and its very very clean and can get plenty loud. DO NOT use two different sized subs it will sound extremely poor.