Either of the following method would work.
1. Create first master string (no need to have a vertical profile) in the median as the master string that defines the scheme chainage/station. Create different master strings for left and right edges, and design your vertical on these. Of course, you will have an issue with projecting the vertical back onto the scheme chainage (due to chainage mis-match) i.e. the vertical parameters will not be exact if you project to a different master string.
2. Create two master stings in the median, which are identical horizontally, but one will have vertical for left median, and the other one for the right median. This may be the easiest for your case.
1. Create first master string (no need to have a vertical profile) in the median as the master string that defines the scheme chainage/station. Create different master strings for left and right edges, and design your vertical on these. Of course, you will have an issue with projecting the vertical back onto the scheme chainage (due to chainage mis-match) i.e. the vertical parameters will not be exact if you project to a different master string.
2. Create two master stings in the median, which are identical horizontally, but one will have vertical for left median, and the other one for the right median. This may be the easiest for your case.