[dorrcoq]

File > New > Multitrack session (a box pops up, fill in the info as needed)
At the multitrack screen, on Track 1 right click Insert > files to load the first source, then do the same for the second source in Track 2
Zoom in the tracks to align them. If they are slightly off, click on the Waveform tab on the top and make an edit (likely deleting or adding milliseconds of blank space so the tracks align). Click the Multitrack tab at the top to return to the original screen and check the alignment. This is kind of a trial and error, might need a few tries.
Do this every few minutes to insure things stay aligned. Make adjustments as necessary.
Make any other edits you want.
When you finish, go to Multitrack > Mixdown Session to New file > Entire Session
When your mixed down file is ready, File > Save As