crossovers on 10dvcs and 15 dvcs

Your biggest problem here isn't the fact that you're trying to use two different sub sizes but that the load will be off balance between the 10's and 15's.  If you have four 10" DVC's then thats 8 coils, then you have two 15" DVC's which is 4 coils.  You have to hook them all up equally like you have 12 coils, not one group of 8 and another of 4 unless you are using seperate amplifiers or outputs.  You could connect the 10's to your left channel and your 15's to your right channel, but then you'd only have the one gain control which isn't a good idea. 

The other option is to design a crossover network that would balance the load with a zobel, but then you're still gonna have the problem of powering the subwoofers correctly to give you decent sound quality.  You'll probably have the 10's overdriving the 15's or visa-versa.

To do what you are attempting the correct way would be seperate amplifers with an active crossover before your amplifier.

Yea, many of us have played with ideas like this before, and it always ends up sounding like like dog crap.

But you can certainly try using one set as subwoofers (<100Hz) and the others as low-bass (100-220 Hz.)  To do so with one amplifier is problematic, and might be rather expensive.   To do what you want to do will require bilding a 2-way passive crossover at 100Hz.  Wire your 10" woofers for a total impedance of 2 ohms, and your 15" woofers for a total impedance of 4 ohms.  You don't have to worry about them matching.  You can use the amplifier LP at 220Hz, then place a HP capacitor array in series with the 10" woofers with a total value of approx. 795 uF, and an inductor made with the largest wire gauge you can find with a value of approx. 6.37mH.  This creates a first order crossover for them at 100Hz.  The parts are likely to cost around $50 for the inductor and around $200 or so for the caps depending on the quality you use.  Or purchase an active crossover and a second amplifier, which would most likely sound much better anyway, as this would allow you to tailor the blend between the two woofer sets and not be "stuck with what you get" like you will be using passive filtering and one amp.

Other probelms besides the crossover include: how do the 10" woofers sound playing low-bass, not sub-bass?  Where are you planning to mount them, since frequencies above the 80-100Hz range are directional and should be in the passenger compartment, oriented to help produce a proper sound stage (meaning ideally acoustically in front of the driver's seat.)  You may also want to employ a low-frequency EQ to help make this frankenstien array sound better, but I really think that while you may think your idea sounds like a good one, you will more than likely be disapointed by the results.