Is it Ok for RCA barells l&r touch each other

I think your completely safe with plugs touching each other? But if your worried, you could wrap them in electrical tape.

In most cases the barrels are connected to the circuit ground and it would be OK for them to touch. In some cases they are insulated and connecting the barrels electrically may degrade the sound or cause an electrical issue.
It should be easy to test with an ohmeter whether the two barrels are connected electrically.

Yes it is ok.
Think headphone mini-plug.

It's not quite the same situation as the mini plug. You are actually creating a ground loop each side of the contact point. Obviously the one on the chassis side is very short. Whether or not the other loop picks up hum has to do with proximity to EMI, and the shielding layout of the ICs themselves.

I've had it cause problems, and not cause problems. A small piece of electrical tape is recommended.

Actually, unless your system is totally dual mono, complete with separate volume controls in separate chassis, then the grounds are tied together. Even in a complete dual mono system, I would be surprised not to see them tied somewhere together.

It is possible, but highly unlikely, that you will have a problem. Keep in mind that stressing those RCA jacks can fracture them and that will indeed cause you some serious headaches. So, be careful with the electrical tape, it isn't two dimensional!

The one time I had that situation I just stuffed a small piece of 3x5 card between them.

I don't know if it made any difference but I slept better because of it.

My harmonix cables do the same on my MA2275, a 1/8 thick corrugated inch cardboard square helped that and also corrected a slight sag from the top cable due to weight.