For me the union is that we are physically together & that twin is part of my life. Although I think that twins create the chemistry that is what union means within their physicality too.
I don't think that it is necessary to have what equates to a physical 3D style relationship as in boyfriend/girlfriend/husband/wife. Although, I do think that some people do have this type of union with their twin.
In fact I know a twin couple that are married. Have been happily together for some time now
