Hello Hezzy and welcome on the forum!
Assuming the two skeletons are already in the same Spine project: (otherwise you can quickly import one into the other) Import - Spine User Guide
You can play two different animations on two different skeletons simultaneously, although if they don't have the same lenght you will see a pause in the editor for the shorter animation.
You can play two different animations in two different skeletons at runtime at the same time.
At runtime, you can use SkeletonRenderSeparator to insert the water in between the glass draw order, depending on the chosen runtime.
Another option is to merge the two skeletons into one. To do so you can parent the bones of the water under a bone, and drag the bone in the glass skeleton.
(this is to simplify things, you don't need to create the bone if you prefer selecting each bone instead.)
The bones can't have the same names.
You will need to re-do any constraint or skin, and animations will be lost.
You can then re-import animations and fix the draw order to be displayed correctly.
Basic Concepts - Spine User Guide: Slots