Hi everyone,
I got a nice black friday deal for a Fender Mustang GTX50 and a Fender Rumble 40 Studio amp. I planned using the first one for the melody and last one for the bass side of a Railboard with R-Block.
Unfortunately when plugging the Y-Jack-cable into the amps I hear a rythmical humming. After unplugging one of the Jacks I hear a proper sound out of the other amp. When plugging in a Guitar or a Bass to the specific amps, the sound is also clear without huming.
So I think it must have something to do the the RB and/or the cable….
Does anybody had made similar experiences? Is this a konwn phenomenon? May it be a problem of the cable (Original SE)?
I am looking forward to any helpul ideas and thoughts.
Thank you in advance!

Edit: Could this be a solution? https://www.behringer.com/product.html?modelCode=P0387

On some of my older amps, the power switch is a double throw.
i.e. the power switch is off, when it is in the centre position.
The power switch can be turned on in either direction.
In one direction, you may hear a hum, in the other direction no hum.
The difference is if the cabinet chassis is hot or grounded.

I can usually tell by running the back of my hand across the metal chassis.
When the chassis is hot, I can feel a sort of electrical stiction on my hand.
When the chassis is grounded, my hand slides smoothly with no stiction.

Because the amp has an AC input, it can run either way, but the chassis will have a different potential. If two separate amps are running, it is possible that there will be a potential difference between the two of them and a ground loop will be created, causing current to flow from one amp to the other. Ideally, both chassis should be at the same potential.

A similar hum will occur when you have multiple input jacks and the ground pin doesn't make a good contact when you aren't plugged into it. These unused jacks have to be grounded or they will act as an antenna and feed an unwanted signal into the amp.

Here's a few videos on ground loops. There's tons of them on the internet.

Thank you very much for your informative reply!
OK, at the moment it seems understandable for me, that running two amps may result in a potential difference when connected over the jacks,
My amps do not habe such a 3-position-switch, so I need another solution.
I wonder if most stickists using 2 amps have the same issues.
Nevertheless I will give the hum destroyer a try, which should eliminate all potential bonding between both stick sides and the amps. It should arrive on monday

Finally, the hum destroyer arrived and works as intended. Problem solved. Thank you!