I'm a keyboard player, transitioning to using MIDI controllers instead of actual synths. I'll be purchasing a USB audio interface for my laptop soon, but I'm worried about the number of separate audio outputs I'll need for a live performance (only using one amp during rehearsals). Is it possible to properly mix two instruments (e.g. lead and pad) through one output, or is having separate outputs for each instrument a must?
In general, the more outputs the better. Having more outputs translates into more flexibility.
You don't give us much details about your live setup. Will you have someone doing the mix for the group? If that's the case, you want two separate outputs and let the guy mix. If you don't have someone doing the mixing, then you will be doing the mix in the box and one output will be enough (or two if you want stereo).
But you might eventually have someone doing the mix, so it's a safe bet to go for at least two outputs for your setup. If you can afford more, go for more. You might want to use more instruments in the future, or add sequences, drum machines, samplers, whatever.
There are cheap, good, and portable interfaces out there with 4 or more outputs, so I suggest you to aim for 4 outputs as the minimum. That way you can also go stereo with both instruments, and there's room for more if you want to add something else later.