To split a track at a specific point, use the Range Selection Tool to select the point(s) you wish to split at and hit delete to split the track at that point.
To extract a segment of existing track as a new track, use one of the selection tools to select the segment(s) you wish to extract, then choose the menu item Selection → Extract as New Tracks or click the knife icon in the tool bar. Adze will create new tracks for the segment before the selection, the selection itself and the segment after the selection.
To merge multiple tracks into one, hold Command and click in the track list to select each track you would like to merge before choosing the menu item Selection → Merge Selected Tracks. Adze will combine all the tracks into one.
If only part of a track is selected, Adze will treat it as being completely selected and will merge the whole track.