There is also a real possibility that your batteries aren't fully charged as you think. You've not mentioned much about your charging or your batteries.

Are you charging them with the supplied wall wart chargers?
How do you know when the RX batteries have a full charge?
How long do you charge the batteries for the airplanes and when do you do that?

Most of the inflight symptoms you describe sound like loss of battery in the airplane or possibly as loss of signal. Interference from other transmissions will usually cause lot's of response from the airplane. You can actually see for yourself what might happen by checking with your equipment. Turn on both TXs and airplanes at the same time. Watch the two airplanes while you both work your sticks. This exercise isn't going to tell you much beyond what happens when there is interference, but it's worth seeing.

It's fairly easy to work out a few things that have not been mentioned.
Right now, you need to test your RX batteries to find out how long they last when you think they're fully charged. It's an easy test and certainly worth doing to possibly save destroying your airplanes.
1. Start off by checking the airplane batteries with an ESV. (You do have an ESV, right?) Write down the readings.
2. Start your watch.
3. Turn on the airplane and the TX. Work the sticks for 10 minutes as if you were flying. Turn everything off.
4. Check the airplane batteries with your ESV. Look at the TX meter. Write down the readings.
5. Repeat 3 & 4 until either a TX shows less than 9.6 or an airplane battery shows less than 4.8 volts.

Now, you know whether or not you've got a battery capacity problem. Until you do that, you really don't know some of the most important things to know.