Replying in flat mode....

Dual masters can be legit. I know I have one in my loft office. It also has a bridge tap before it, so fun for FTTC installer when it finally happens.

I'd have expected this

1. One line left untouched still served by one socket in the old dual master socket
2. The other line sent to a new nte5a next to old socket and VDSL faceplate fitted, i.e. allowing you to test at the master and any extension wiring rearranged onto it.
3. If there was a lack of slack then it may have needed some jelly crimps and a short run of wire perhaps.

Photos are needed and if ISP is not playing ball and it looks like a bodge then am happy to push it past the usual layers on your behalf to get a tidy up visit arranged.

You have misread the entire OP.

The OP has dual master sockets with faceplates, not hard-wired master sockets.