My suggestion would be to use a different approach.
The float material
There are adhesives sold for repairing inflatable PVC boats. They do a great job, on a boat. But they may not be ideal for a pool toy.
High-quality inflatable boats are made from a coated fabric. The fabric provides the strength and the coating makes it waterproof and air-tight. If they need repair, a permanent repair uses a patch of similar material. The adhesive makes a permanent bond. Afterward, you still have fabric for strength, and a waterproof, air-tight surface. What happens to the original coating doesn't make much difference because it is then just an internal layer.
Moderate-quality inflatable boats can be made from just a thick sheet of PVC. The seams are typically heat-welded. If it needs a permanent repair, you would normally use a patch. The situation is similar to coated fabric. The vinyl is thick enough that the adhesive can do what it needs to do on the surface and you still have substantial unaffected vinyl below it for strength.
Cheap vinyl pool toys are made from a relatively thin sheet of PVC. It has enough strength to not burst. But whatever strength it has comes from the intact vinyl. When you glue vinyl, many of the adhesives that make a permanent bond with it affect the physical characteristics of some thickness of it. With a cheap pool toy, that is likely to be much of the thickness of the vinyl. So there would be very little thickness of vinyl for strength, and it may be prone to bursting in that area.
If you use a patch, the adhesive would go to the edges of the patch, so the border of the patch could end up a weak area. If you glue two of the toys together, each is a patch for the other, but you could have weak areas around the border.
The specific inflatable you linked to didn't get very good reviews. Some of the many 1-star reviews describe the material they're made from as cheap, light-weight vinyl, which wouldn't bode well for gluing. This question asks about permanently connecting them, but the lifetime might not be very long. And if they're permanently connected and one unit irreparably fails, you lose both.
The shiny PVC in pool toys can be hard to adhere to. The stuff that works well and retains flexibility generally affects the vinyl. PVC plumbing cement would make a chemical weld, but it would affect most of the thickness of the vinyl. It could make it more brittle or weaker. It might not, but finding out would be an expensive test.
There are some adhesives that bond with the surface but don't affect it to a significant depth. They're sold as part of a patch kit or separately in tiny tubes. It would get expensive on the scale of your project.
If it was my project, just on the basis of the product reviews, I wouldn't risk two inflatables by permanently joining them (even if you could do it inexpensively for the adhesive and at low risk of the adhesive weakening the wall).
I would tape them together with waterproof duct tape, which you could get in a matching color or a camo pattern. Apply the tape to the clean, dry inflatables, and do it when you are ready to put them in the water. Wipe the surface down with alcohol, and inflate them to normal pressure. The sides will balloon out so you can tape them where they will be in contact when inflated, and you will be applying the tape to the stretched surface (this is much harder to do with adhesive).
The tape may not last a whole season. Just apply new tape as needed. If you deflate them, the stretched vinyl will shrink again. Repeated deflation and inflation may loosen the tape's grip. If it comes loose, replace it. The amount of tape used would be pretty inexpensive, and applying it doesn't take much time.