Hi all, thanks for the further advice, I actually didn't have a chance to look into this more. There is good reason I don't want the second battery to drain at the same time - it will be acting as a UPS.
Basically I run computer kit often away from mains AC for long durations, I am looking to make an 'all in one' DC power distro and UPS. AC should be the primary source of power, with an exterior battery the second source (so the distro would switch to the battery if AC is pulled or not present). However when running on the external battery, if this battery runs out or fails I need there to be a backup to switch to - hence not wanting to drain these batteries at the same time. This will also allow the main exterior battery to be switched out without AC power and without losing power to the system.
Here is a rough diagram:
*I would note that total power consumption will be significantly lower that the upper end of what I am speccing it for.
In an attempt to keep this 'simple' and try to steer clear of circuit design which I would have to pay someone else to do I think... I am considering just have a 13s2p 18650 bank, but limiting the charge voltage to 40-50v, I would lose some capacity but would still be enough for switching power sources (as well as being high enough to allow the DCAC to work), and would work with the 'off-the-shelf' ideal diode circuits as its voltage range would consistently be lower than either primary source.
Although I have been looking at the Analog/TI websites, I couldn't find anything that seemed suitable. It is well possible that I am missing things based on lack of understanding of the terminology in the data sheets.