Can a 3.5mm headphone splitter be used to join two inputs into one output?

Solution 1:

I would not recommend that. Any time you join the outputs of two or more amplifiers (and a soundcard is just that, if weak), you risk damaging one or the other.

Solution 2:

You could build your own solution if you are looking for a fun project for the weekend.

Solution 3:

The other way is also possible, use for example 2 inputs and 1 output (with a switch): http://www.instructables.com/id/How-to-Construct-a-35mm-Switch-diy/

Please, check the picture for my own result of this project: