MK7 flat battery question (Twin batteries !)

[HARRYSPADGE]

Just heard the alarm going off on my van, the alarm is a Cobra 3866.
Went out to it and with the key in the ignition and switched on, found the battery completely dead.
Realised I'd left the lights on in the back which had drained the battery, they are additional lights I have wired in to the connection points by the drivers seat.
Now being that my van has twin batteries I thought that if one went flat, when you turn on the ignition it causes the second battery to kick in ?
Which battery do the connection points by the drivers seat run on ?
The only way I could get any charge in to the batteries to start it was to have the jump leads connected to the jump point under the bonnet and have the ignition on so both batteries were engaged.
After about 10 minutes of charging from the jump vehicle I managed to get the alarm switched off and the van started.
Can't get my head around how this happened, I always thought that if the front starter battery went flat the second battery would provide starting power !
The batteries are both Bosch which were replaced about 2 years ago so I'm pretty sure it won't be a battery issue.
Any thoughts guys ?

[blue estate]

front battery does locking lights and powers realy which connects back battery so you can start

[HARRYSPADGE]

front battery does locking lights and powers realy which connects back battery so you can start

I thought the front battery was the start battery.
If the rear battery is the start battery then I can only assume that the front battery powers the connection points by the drivers seat and hence has gone flat due to the lights being left on.
This would mean the battery was too flat to power up the relay which connects the start battery ?

[blue estate]

front battery does locking lights and powers realy which connects back battery so you can start

I thought the front battery was the start battery.
If the rear battery is the start battery then I can only assume that the front battery powers the connection points by the drivers seat and hence has gone flat due to the lights being left on.
This would mean the battery was too flat to power up the relay which connects the start battery ?

Yes yes and yes

[HARRYSPADGE]

Thanks Blue, just got my head around it now.
Had it all A**E about face