Merge Vertex Groups

Unfortunately, i had two bones within the pelvis of my character when i set bone heat; one bone owns the vertexs at the front, ie, navel, and the other owns the arse vertexes. I was wondering if there’s a handy way to merge the groups, while keeping the various weights. With the way the bones are right now, there’s stretching when they don’t move together, which is problematic.

Any help would be appreciated.

Could this be solved by using a extra bone and Constraint?
I’m not so good with understanding “groups”… :o

By ‘groups’ i do mean vertex groups. My apologies for the lack of clarity.