Ive got 2 600mah receiver batteries. I know i can connect them series or parallel, and be like a 1200mah battery right?

You can connect them in parallel, but NOT in series. But in general, this is not a good idea. A better way would be to use a circuit that will switch over to the second pack when the first one runs low.

You can use two batteries really easily. However it is usually only the turbine and giant scale guys that do this.

I would use one battery and save the weight.

Geistware had a great diagram on his website www.rcaircraft.net It really helped me alot on the same issue.

Just make it easy to get into the trainer so you can swap the batteries.

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When doing dual (or multi) battery systems... you need one switch harness per battery. And you need to either plug the batteries int separate "channel" locations on the RX or use one of the optical isolator devices.

This is fairly common on large models with more than 7 servos.

My opinion is... if you are going to the trouble of doing dual batteries... you may as well add the extra 1.5 oz of a 2nd RX to have redundant control of the aircrat. Tests done with these systems show that with one aileron, one elevator half, throttle kill (one RX has kill only... the other has the proportional control) and no rudder control, (this is a worst case of a proper setup.) you can control where the model lands and almost always can land with no damage. When the model has damage its definitely less than if all control was lost...

What size plane? If this is .40 - .60 size plane it's really not worth adding another 1/4 pound to it.
As stated, batteries can't be connected in series or it will double the voltage.

To amplify on previous posts, it's in parallel. This (parallel) will not change the final output voltage, but will increase the capacity to 1200mAh. If you connect them in series, your capacity will still be 600Mah but your final output voltage will double, no doubt letting the very important smoke out of your RX and servos. You'll have to send them back to the factory so they can put the smoke back in.

I agree that joining two batteries just to increase capacity is a bit of a waste. It would be better and safer to just get a larger pack. Many of us (myself included) run dual battery setups, but they are part of an entire dual setup that includes redundant switches and RXs. Such a system is overkill on first or second models.

There is nothing wrong with using a small pack, as long as you check you battery before every flight (something one should always do reguardless of the size of the pack or when it was charged.) As long as the pack can handle the current draw demanded, it will be fine. 600mAh will work fine for planes up to 1/4 scale. The advantage of a small pack is weight savings. The main disadvantage is that you will get less flights per charge.

If I am not mistaken, the problem with over voltage is with the servos and not the RX. Ann Marie and BAX can answer this more thoroughly but the receiver can handle a lot highter voltage than the servos can.

I think most receivers can handle up to 10 volts but anything above that is risky. Two 4-cell packs that were mistakenly connected in series could ruin the Rx as well as the servos.