Two 15's is too much ?

Depending on how you wire the subs, if you're using the same amp and just adding subs, there should be no issue if the amp is drawing the same current from your electrical system. If you're adding another amp, or dropping the impedance load on the one you have now, then you could draw more current and tax the system...it is probably wise to stick to a "not to exceed" current load on the stock electrical system....some (here on PC) have suggested 100 Amps of current draw will not pop the fusable link, but I'm not sure exactly what that threshold is. Two 15's seems a little excessive to me, but I went through a "bass-head" phase myself, so I can't really criticize. If it were me, I'd be seeking ways to get maximum output out of some smaller woofers with smaller enclosure options (maybe three 10's?). Two 15's, in the enclosure volume they'd like, would take all of the cargo and back seat area.

 

may I ask why??? (for ref I have a avatar 2x12" bass cab and at 50% it rattles whole house)

 

"Excessive" in that the enclosure for two 15's takes up an impractical % of the space in the cab of a Prius. The output is a matter of preference....my point was that it seems wiser to get the output he's after through a combination of smaller drivers in smaller enclosures...which will require more power.