So i have a mixer with 4 mono aux-sends, and i want to find a simple way to convert the signals to "stereo", so i can hear the signal in both sides of my headphones.

I found this cable and wondered if that would do the trick?

I don't know much about this stuff, so i was wondering if there would be any problems with using this cable.
Thanks in advance and sorry for bad english, hope you can help
well i want to connect the cable to a patchbay, which i can then plug my headphones into, so wouldn't that work?
Is the patchbay stereo?
I'm betting that it isn't, in which case fly's suggestion is the correct one.
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Stereo is not balanced it's 2 channel unbalanced. A balanced signal means the hot and return are isolated from ground unbalanced means the return signal is grounded. TRS jacks support balanced connections through an XLR Plug and mono unbalanced connections from a guitar 1/4 inch jack (aka 6.3 mm).
Depending on your setup and device you may simply be able to send the mono signal to both channels (left and right) and so hear it in both ears. An example is USB recording interfaces, you plug a guitar in you can either have it in both channels or a single side.
Moving on.....