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: