You need a “3.5mm Stereo Female to 2-Male Y-Splitter Audio Cable” or more ideally a four channel line mixer or two channel stereo line mixer.

This works with splitters (and you can combine cables and splitters to get there, it doesn't need to be a single y cable with the right ends). I'd recommend against it however. Two outputs for one source is usually fine, but two sources to one output with just a y splitter can be detrimental. Depending on exact circumstances sound quality can be worse and/or it can (theoretically) damage your equipment.

For two low level sources it's probably fine in most cases, but definitely not recommended. There are positive summing circuits that prevent this, but usually the recommendation is to use a mixer instead.