splitting a water heater bill
No, no, no!  Not only would it totally not do what you want, but would make the cost splitting even worse, as one tenant's other load would wind up on the other tenant's bill, simply due to the way electrons seek the path of least resistance. 

It would also create neutral and ground loops that would make heat in unexpected places, and your circuit breakers wouldn't protect you from that. It could burn the building down. 

A less-unsafe way of doing that would be seek out a water heater with two separate heating elements on 2 circuits.  For instance larger on-demand heaters have this.  Put one circuit on one service, and the other circuit on the other service.  If they operate together, that would split the cost.  Check with the manufacturer to see if they do that.  However there'd still be an issue of ground.  If the apartments are served by 2 service panels, each getting their grounds from different locations, then you would be entangling grounds from two different systems, and my gut reaction is code would prohibit that.  If there's an exception to be found, or a way to rejigger the panels' grounds to make this safe, a proper electrician could tell you better. 

In the apartment business, the normal way that's handled is by a third meter for commons areas.