You can put any song files anywhere on the timeline in any order and arrangement you wish. You then add track markers anywhere you want. There's no need for the track markers to fall between songs. You could put 15 songs on the timeline and then add just one track marker at the beginning (with it's corresponding end of track marker at the end) and you'd get one single CD track containing all 15 songs.
So, if you want two songs to be combined into a single CD track, simply don't put a track marker in between them.
I've never used CDA, so my experience is with the CD burning function of Vegas. However, they should be nearly identical. Try right-mouse button clicking on the track marker and choosing delete. You may have to delete the start and end markers individually.
Also if you right-mouse button click in the thin horizontal area where the markers reside, but not on a marker itself, you might get a popup menu that lets you delete all markers.