Basically... you press the switch, which allows the POS to activate the MOSFET. The (active) MOSFET allows the NEG to reach the REG which activates the REG. The POS from the REG goes to the 510 center so the 510 now has 5V regulated output ONLY when the REG has been activated by the MOSFET. The GND from the MOSFET to the REG should also go to the 510 GND. Now the 510 only has GND when the MOSFET is activated.
BATTERY POS -> SWITCH + REG INPUT
BATTERY GND -> MOSFET
MOSFET -> SWITCH + REG + 510 GND
You can NOT tie the BATT GND and MOSFET DRAIN to the housing or that would bypass the MOSFET... so... since your 510 will likely be making contact to the housing (if you use a metal housing), I would suggest having the MOSFET DRAIN tied to the housing. In this case, the GND used in the REG can be tied to the chassis/case/enclosure (if metal). Anything grounded to the housing in this case would only be connected to ground when the MOSFET is active. This only applies if you are using a metal housing, but I thought it was worth noting.
NOTES:
Don't forget any CAPs needed for the regulator to smooth output etc.
Don't forget any resistor etc for the MOSFET in question, but remember you are now feeding it 7V - 8.4V depending on the battery state.
It would be wise to include a LOW VOLTAGE LOCKOUT circuit to prevent the device from being able to drag the batteries below whatever the safe lower limit is of your batteries or the system from drawing above the safe AMP limit due to low voltage levels.
Don't forget to add fuses just in case there is a direct short on the batteries for any reason. Also, read the fuse datasheets... i.e., a 12A fuse takes a specific amount of time to blow due to 12A.
If the MOSFET is 95% efficient... and the REG is 95% efficient... you have a total efficiency of about 90%. This means that if you are pushing out 10A to the atty, then you are pulling in 11.1A from the batteries and the extra 1.1A are being turned into heat at the REG and MOSFET. In this scenario the MOSFET would be wasting about 4W to heat and so would the REG. All of these numbers depend on your real parts, voltages, etc. So, just remember to base your batteries on their ability to safely release enough AMPs for the input of the system, not based on the output of the system.
Is this where I mention vent holes
My suggestion: Since you are using stacked batteries... you have a [somewhat] unique opportunity to include a very simple kill switch at the point where you connect the two batteries together. That connection could be a removable slug... basically you kill the whole system by pulling a tab out which disconnects the two batteries from one another. Basically, the slug could normally be what makes the connection, so when removed, they are no longer connected.
EDIT: Oh yeah... look into the various safety requirements around to using stacked batteries and remember that you may want to fuse them individually [as well] since one battery will end up failing before the other one due to slight manufacturing differences, if for no other reason.
