OK, both castle BECs are rated to 6S, so connecting the BEC over both batteries is 100% OK. It is also OK to connect to just one battery (the ground side battery).
Now, as for connecting the ESC BEC and the external BEC.
If you just connected the External BEC at 7.4V to the RX AND the ESC BEC to the RX at 5V it WON'T be 12.4V. What will happen is the external bec with try to power the ESC BEC and burn it or BOTH of them out. So you can't connect 2 BECs at a different voltage ever (not really recommended to connect 2 at some voltage either unless you know what you're doing, it has to do with the style of BEC, either switching or not, but that doesn't matter in this case).
So now since you don't want to use the ESC BEC anymore, and you can't just turn it off (standard for ESCs). All you have to do is remove the red wire for the lead going from the ESC to the RX. All that red wire does is give the RX and consequently the servos and anything connected to the RX the 5V. So removing the red wire (or using an extension with no red wire is great since you don't damage anything in case you need to use the ESC BEC again) basically turns off the ESC BEC.
Now the tricky part with your situation comes in with the high voltage servo. If you don't have a high voltage RX you can't run 7.4V to it. In this case you'd use the ESC BEC to power the RX by leaving the red wire connected. But now you can't just connect the external BEC to the RX since it already has voltage. To account for this, you would wire it up as in the first photo shared here.
BUT, since you have a RX that can handle the 7.4V (I DON'T KNOW IF YOU DO, YOU need to verify this yourself). It makes it easier since you can power the RX and servo from the same supply, the 7.4V from the BEC.
And since you have a castle BEC, it's good up to 6S lipo, so you can connect across the whole battery connection.
So, simply wire it up like this (if you have a high voltage RX that can handle the voltage you want the BEC to put out).
Hope I explained enough, please post back with specific questions if you have anymore, vague ones are pretty tough to answer. :D
Edit: Just noticed you said you already wired it like the diagram above me. That is totally fine too. As long as you have a RX that can handle the voltage you want the BEC to put out (sounds like you do). Looks like you're all set! Just make sure that the BEC ground is on the ESC ground as well. I hope my long post at least explained better as to why the wired it is OK.