I was recently asked to make a "Y" cable... Male XLR to 2 Female XLR's to be used on the camera so 2 microphones could be combined into 1 of the cameras inputs.
Now while technically possible it needs to be noted that doing it this way the background noise will increase and it gives NO control of individual microphone levels, and should NOT be used for critical audio recording. This is where mixers come into their own giving control of levels and dropping unused microphones.

I have two "Z" cables (2 Male & 2 Female XLR's in a Z configuration) in my kit I find them much more useful as they can be used as a Combine, Split, M-M or F-F cable.

You can have difficulties combining two mics with a simply Y cable as they can interact with each other. Far better to use a simple mic combiner with a transformer that isolates the 2 mics from each other. An example is this one from Whirlwind .. Whirlwind / Combiners (http://www.whirlwindusa.com/comb.html)