Moin dropshot,
as I understand Your Y-cable, it takes two Mono signals and puts them into one Stereo output (if driven in 2 -> 1 direction) or splits a stereo input into its Mono-components (if driven in 1 -> 2 direction).
This will probably not suit Your needs: Even if You will get along with cables, You will need one
Mono-Y-cable for every single line that has to be "doubled".
My personal advice however is to use a little mixer. Simple mixers with 2 stereo-inputs and one stereo output will cost You less than 50 Euros (here in Germany at least). They also drop in a headphone amp and a microphone preamp. Of course You will have to live with the knowledge that chinese kids made it for You, but this is very hard to escape in general today...
https://www.thomann.de/de/phonic_am_55.htmhttps://www.thomann.de/de/the_t_mix_mix_502.htmThe
real solution might be a more sophisticated mixer, allowing You to route different stereo or mono signals through different effects in different levels, but this is another story and You should try to learn about mixing before You select what to buy.
Go for that 50 Euros!
HTH
Martin