You have the potential to put out 7000w or more from those 2 amps. The 5k can draw up to 500a on its own and about 250 amps from the 200.4 Realistically, you will probably see about 500a of draw between the 2 amps. I would expect voltage drop into the low 12s with your current electrical. I would definitely recommend a second pair of XP3000s in the back.
When trying to figure out how much battery power I need to setup a system, I usually like to have twice the amount of battery power as I have total wattage. IN your case, you have potentially 7000w, so I would want 14000w of battery power. The XP3000s are rated for 3000w of power support, so 4 XP 3000s + the D3400 under the hood should give you substantial support to go along with the 270a alt. I always try to take the alternator out of the question when figuring how many batteries are needed since the amps are run off of the batteries and not the alternator. The purpose of the alternator is to keep the batteries charged and not to power your amps.
Here is another way to tell if you need more batteries. 12v battery resting voltage is about 12.9v, once your alt kicks in your voltage goes up to 14.4v. If you have voltage drop down below 12.9v, now you know that you have maxed out the amount of amperage that your alternators can supply and are getting into your batteries reserve power. Keep adding batteries until your full tilt voltage stays above 12.9v. The more battery power(reserve) that you have, the longer you will be able to play before you have to turn it down and let your alt recharge your batteries.
Hope this is helpful.