JohnMac

I agree with you Dirtybird. some time ago a freind decided to put a second 4 cell NiMh battery in his first jet. My advice was to replace the single battery with twin, new batteries and isolate them with diodes. However, he had read in a UK mag that this was unecessary, because everything works just fine if you plug two batteries into your RX. The writer startted his article by pointing out that he had no knowledge of electronics! And of course the writer was quite correct, when everything is working, everthing is working. All is well with the world.
But we do not build redundant systems for when everything is working but rather for when something goes wrong, and in order to test if your redundant system actually works, you need to put the system into the failure mode it was designed to overcome.
My friends jet burst into flames when he switched on the second battery. Of course we had a fire extinguisher on hand and the fire was out in a few moments. It was not the battery that had shorted out but the switched that it was wired to. The leads on that switch were blackend, and the terminals of the switch were welded together. This had discharged battery 1 whist it was switched off, as it had a negative lead running between the terminals. II suspect the the insulation on this wire thad been mechanical worn away. In all probability this had been a "slow burn" that had turned the switch into a lump of carbon, but then when he switched on the second battery, that battery was presented with a short which quickly heated the wires to the point that the insulation melted, creating an even better short, and a fire. It is for this failure mode that we use diodes. A little bit of knowledge can be a dangerous thing.
However, I would caution you against using two batteries in the way you have suggested. If you use one for the Rx and a different battery for the servos, you are now relying on two batteries, switches and harnesses to be good. That is a worse situation than having a single battery. Better to have 2 batteries in parrallel, either through some propietery power distribution hub, or through two diodes, one in each switch lead. In this way you have double the capacity of battery on tap, and double the current availibility too. Best of all should one system fail, you have second. Bigger capacity and better suited batteries are they way to go, and always two of them IMO.
Finally, NiMh are for TV remotes!
John