Curve Offset split

 From:  Lemo (LEMONNADO)

I have a curve which is a join of two curves. I offset that to one side and it splits open. As of the joined curve is sill consisting out of two. The problem is that the two new offset curve parts are not 'fitting' to the original curve. They are offset in 3dspace as well to each other and a simple 'patch' ruins the continuity.
How can I join two curves so that they are not 'split'?