Sometimes our customers need to couple dissimilar plastics tubing. Most often they couple provista (PETG) tubing with butyrate (CAB) sleeves. We recommend this technique. It is simply using the molding shrinkage of a heated tube to form a compression coupling.
After inserting the inner tube into the outer tube slide and clamp the hose clamp. You can keep the clamp on, or after cooling remove it (it will leave its impression).