Is it safe

Not a good idea. Thee output of one of the items connected will 'see' the output from the other as a voltage coming in..
Most audio product outputs do not do so well with a strange voltage entering the output transistor.

So in general no it is a bad idea to connect to outputs to the same input (Even though one is turned off) for the above reason: the voltage is presented to the output stage of the turned off item.. and that is usually bad.

No, not a good idea at all. Use one or the other, not both.

Also it says to NOT DO THAT in the 502P owner's manual on page 8, section 1. Distortion will occur. Really...that's what it says.