How to prevent from changing volume when using 2 to 1 RCA connector
It is dropping in level because the end of an audio connection usually has a termination point. and/or the signal is being routed to electronics that are adding slight resistances to the signal.  (in simple terms there is a cap on the end :-)

The least solution, would be to use an isolation transformer to one of the connection point, then one of the Yed signals could be isolated from the other device.

The most solution would be to use an audio distribution amp. Or to  route through a device that would passthrough the audio (unterminated) Like some amps or pre-amps. or to use other devices (like a equaliser) that would have 2 seperated outputs.