I'm not sure I follow you here. There will always be either one large span or multiple ones and in the latter case they do not exactly merge, only their temporal boundaries eventually will coincide at one frame (their unmerged status being denoted by the arrows pointing in opposite directions still). The default span length is 20 frames, so clicking outside that range will create a new base frame and thus a new span. Only moving within that threshold will cause analysis and extend the current span, infinitely, if you so desire....
Basically I think, I want to freeze one and work in another, but it doesn't want to let me do that.
Roto Brush doesn't work that way. You can set the range by dragging, and it will analyze from both directions, but if you go back to say frame 20 from frame 200, it will analyze from 20 all over again.
The most efficient way to use Roto Brush is to start at the head of your shot and go forward until the track starts to fail or until you need adjustment but stop before the end of the range. You then add a correction and start analyzing again. Proceed with this technique stopping every few frames to make adjustments, then freeze, and if you're smart, render the just processed footage with an alpha channel and replace usage in the composition. This will keep the AEP file size smaller, make your life easier, and be the shortest process in the long run.