Well there's a thing called box rise or impedance rise. If you don't know about this, I suggest you look it up. Generally small sealed boxes have less box rise than a big ported box so it would make sense that your sealed box is getting more power than your ported even though you haven't touched anything on the amp. You could also be clipping and not know it. How did you set your gains?
Two idmax's need sufficient power especially in a sealed box so I would suggest you upgrade your alt. I would suggest you get a bigger better amp in the future too but in the meantime you'll be fine. I totally feel the sentra's alt is not up to par with what you're running. Your front batt is good. Adding a 2nd battery would just be bandaid for the power demands you're asking for.
I'll tell you about my horror story with that exact car you have. Not only did the battery die on me several times, but one time it was a hot summer day and I was driving around in the middle of traffic. I was blasting the A/C and as I stopped at a red light, my car died while the car was IDLING! Piece of sh1t alt couldn't even charge enough to keep the battery alive. I was stuck at the light with people behind me and everything. I had no music on and everything is completely stock and new. I kept turning the key and the battery somehow revived itself after a bit and car finally turned on and I turned off the A/C. The A/C isn't even that cold either and the voltage keeps dropping up and down like crazy. This is why I'm saying you should upgrade your batt AND alt. $270 for a 170a alt is not bad really. I paid like over $400 for a 160a singer alt for a corolla. I say go for it if it's still for sale.
The fucked up part was I had jumper cables in my trunk and I couldn't even open the trunk because there was no power! I have since replaced the battery with an interstate battery and it hasn't died on me. The A/C still fluctuates the voltage though.