You can damage gear this way. A splitter cable is for splitting, not joining. You can’t use them to sum (not correctly anyway).
The outputs are low impedance and the inputs are high impedance. Connecting two outputs to one input can cause current from the outputs to go “backwards” to the other output, because that’s the path of least resistance.
To sum two stereo signals to mono you either needs something designed for that (e.g. a mixer) or a summing cable which has resistors in to stop the outputs acting as inputs.
Edit: thanks to @llatsni for the link in this comment:
And this is the one where I first learnt about it (which links to the Rane one):
http://silentsky.net/wordpress/archives/624