I haven't seen the chapuis / verney-carron settup but I would imagine that each underlug is closer to the centre line of each barrel so there would be less tendency for the barrels to pivot left and right compared to the traditional double having the lug positioned in the middle between the barrels?

Demi-bloc is another name for chopper-lump so your Aya and your Bruno are both chopper-lump.

Heym use shoe-lump barrels on their double rifles. You might find some pic's or promo video on their site or if you google it. The Heym rep posted some good pic's over on AR showing some of the machining and jointing process.

Also there is a poster on here by the name of Alex Beer. He was Australia's very own double rifle maker. If you do a search you will find some pic's and info he posted on jointing shoe-lump barrels. I prefer it because you end up with a nicer profile on the barrels. A nice straight section around the chambers for the length of the water table before the barrels start to taper down to their smaller diameter. If I ever have a crack at making a double I'll be trying the shoe-lump method.