ok well ill take a stab at it. and yes your question makes sense.
subs have a recommended power rating, that they'll sound their optimum at. i dont know what it is for the ID MAX. you have a point, that the power will be split 50/50, the issue is, will you still be powering both subs optimally.
doubling the subs = doubling piston area = louder. but only if the amp can drive them.
find out what the recommended power is for the ID MAX, if its 5-700, then go get another one and pound away. but if its 1000+, then you'll be underpowering them and while they may be louder, they wont sound as clean/crisp.
hope that helped!