Once the edges are ligning up properly go ahead and glue it up. Use the appropriate cement for the material you are using. There is a cement for PVC and a cement for ABS. If your 1.5 inch pipe is different material than your 2 inch, use the proper cement for the 2 inch pipe. The cement typically comes with a 'brush' attached to the lid. Take and generously slop the cement onto the pipe. Try to get good coverage on the edges of the cut where they will butt up against each other. Don't worry that it is dripping all over the place inside the pipe. This will just help add character to the inside, much like termite trails would on a euc. Once the cement is applied tighten all clamps... make sure you do not forget the 1.5 inch piece at the end. This piece should be slopped with glue on the outside circumference. I always apply cement to the inside of the 2 inch pipe as well where this piece will go. The end of this piece is to be cemented flush with end of the 2 inch pipe as shown in the last picture here.
Let this whole mess dry for at least 24 hours. NOTE: If you are tapering a 3 inch pipe the cutout should extend most of the way down the pipe for a less dramatic taper. The clamps may need to be tightened in sequence working up and down the length of the pipe a little at a time. The tension on the pipe and the clamps has a potential of being very great. A trick I learned was to heat the pipe up with the torch directly opposite where the cutout is to help relax the springyness of it. This is strongly recommended. Be careful not to heat to close to the cutout as it may distort the edges to where they don't align properly.
WARNING! The heating of these synthetics will produce toxic off gases. One should take proper precautionery measures to ensure their own safety!
You are now ready for Heating/Bending.
Construction of the PVC/ABS Didjeridu