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OK, share topo connects the volumes, so rather than having two faces at a volume contact you have one. That gives you a linked mesh, with nodes/faces shared between the volumes. That'll be covered in some of the courses under Learning. 

Multiphase is a fun area to get into. We assume that all phases flow, so solids are granular and behave as fluids with some extra models to account for particle contacts.  In your case, with phases mixing the whole domain should be mixture. You then patch zones to be a single phase (sort of - read the model theory) to give the starting point.  If the hydrogel disolves the calcium you have a different problem set up which may or may not need a completely different approach.