Merge two GTM sync-groups into one

Business wants to merge the two separate DNS systems into one by making one common GTM sync group. The sector A has three GTMs and their respective LTMs. Sector B also has three GTMs and a few LTMs. Both sectors maintain their list of Wide IPs, some of the them on the common domain while most of them are on separate domains.

The requirement is to merge these two sync-group without overwriting the existing wide IP lists and other config. After the merge, the expectation is to see the Wide IPs of sector B to appear in Sector A and vice versa. I have a few theories to accomplish this, but, wanted to come down here and seek for your opinion and see how would you like to do it. Any help is appreciate.