If all you're doing is using a mono out on the drum machine and a mono out on the synth, then you will have no problem. It works just as I decsribed earlier (unless I'm completely missing your point). At least I'm at home now, so the following description is VERY accurate and detailed, unlike the one I gave from work the other day.
First, get the right kind of cable(s). I'm guessing your drum machine and your synth both have a mono output, probably your left output. I'd also bet that these are 1/4" mono jacks. You therefore need a cable that has a pair of 1/4" inch mono plugs on one end, one plug for each instrument. I have an SB Live! Value, but I'm guessing your card also has a single 1/8" stereo jack as a line in. The other end of your cable would therefore be a 1/8" stereo plug.
Next, start n-Track and click on the "Input VU-Meter" button. When the meter pops up it will actually say "Recording VU-Meter", not "Input VU-Meter". Click on the "Settings" button. You'll get another small window titled "Select Audio Format". One the left side are your selections for "Channels". The button selection is "Stereo -> Two Mono Tracks". Make sure that is selected, then click the "OK" button. You are now set.
Just in case you've modified (accidentally or not) your menu bars and no longer have the icon across the top, you can also get to the Recording VU-Meters another way. Click on "View" from the top menu bar, then select "Recording VU-Meter". it will pop right up.
Now when you record you'll get two mono tracks. One will have the drum machine and one will have the synth. Note that these two tracks are both panned to the center by default. To pan them L/R to suit you mix, You simply turn the pan knob on the channel strip for that track. You can do this one of two ways. For the first one, find the track you want on the n-Track screen. There will be a "M", an "S", and a ">>" arranged vertically for each track. Pressing the ">>" gives you the channel strip for that track. In the middle on the left side there is a "knob" with a single number below it. It will probably be 0 initially, which meanas the track is panned dead center. Click on that knob BUT DON'T RELEASE THE MOUSE BUTTON YET! As you move the mouse left/right while holding the button, you'll see the knob adjust the pan setting. -100 = hard left, 100 = hard right.
You can also get to this channel strip from the main mixer window. You modify your setting the same way as on the individual channel strip.
That's probably more decsription than you need but I wanted to be sure you understood it. I apologize if I insulted your intelligence.