[Vim Fuego]

Try either removing the earth connection from a jack lead and using that in the noisy amp - like Keith's diagram above but without the earth, just the top wire in the diagram.
Or try putting an isolating transformer/di box between the gt-8 and noisy amp.
Jim

 

[Vim Fuego]

Take an ordinary guitar lead, cut or preferably de-solder the connection to the ground on one side.
Like this....
The trouble is at the moment you have two ground connections to the two amps, this will eliminate one but you won't be able to use the cable normally unless you re-make the ground connection!
Jim