Sharing an amplifier audio input with a Y-splitter ?

[John_Edward]

tl;dr:
Can I use an Y-splitter to connect a Turntable and a TV to the same input on a hifi system ?
Should I set the Turtables pre-amp on or off ?

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So, my grandmother has the Onkyo CS-254BT hifi system, which has just one 2xRCA audio input at the back, which is currently connected to her TV (using the TVs headphone connection)
I happened to get a Sony PS-J20 turntable I'm planning to give her for christmas.
This turntable has an internal pre-amp you can select from the bottom.

If I get/make a Y-splitter cable to connect these both to the same input, would it work ?
There is a real possibility she will try to use them both at the same time, will this break something ?

I know there are inexpensive selector boxes available, but if I get her one, she _will_ call me twice a week asking why her TV doesn't have any sound, so...