Bramble,
I have seen the same thing that Ron described happen on a rifle here.
It was regulated using a similar device and when first done shot perfectly, but after it was soldered up it all changed then shot 12 inches apart at 50 yards.
The guy who was doing the work had regulated a couple of other double rifles that he had built the same way and it had worked well.
This all worked out real well for me as I was offered the rifle at a price too good to refuse and ended up rebuilding the barrels and fitted a wedge and regulated the thing in the traditional way.
That gun is one of my main shooting doubles and has taken a lot of game.
The thing I like about the traditional method is that once you finish the regulating and have cleaned up the end of the wedge and any excess solder, you're done apart from the rust blue job and you know it's right.
Not trying to talk you out of giving your device a try though, it may work just fine.
Only one way to find out I guess.