Best method to key in new Gypcrete

[highup]

I measured areas to repair in a large restaurant kitchen. Two damaged areas need new Gypcrete brought back to the level of the existing. It will then receive new gray quarry tiles.
How do you prep the edges where the existing Gypcrete meets the new?
Does one prep he edges where they meet with the "correct" primer and call it good
.........or is it possible to shape or profile the edge to stabilize the joint?
The larger area is just 12 by 6 or less. It will be one and a half inches deep.
The substrate is plywood.

 

[MSLI]

Do they make cement board 3/4” thick ?
Maybe two layers of that for the open space and then prime / patch the gaps with gypsum based material.

 

[havasu]

Do they make cement board 3/4” thick ?
Maybe two layers of that for the open space and then prime / patch the gaps with gypsum based material.

Not at the local big box stores, but they do carry 1/2" and 1/4", so they could be sandwiched.

 

[highup]

Naw, wouldn't want the two materials to be structured differently. One might be more rigid than the other and cause issues.
I tried to get ahold of the company rep but this is the second time in 4 months that he hasn't replied. I'm thinking he moved on.
He knew every product like the back of his hand. Very helpful guy.
I'll call the company tech line and see if the guy that answers is good or just yesterday's shoe salesman reading from a screen with no idea what he's talking about.