Always On: replication over cross server SQL instances I do agree, the question is not very clear. Let's explain that.

Context: we are wondering if it's a good idea (regarding our business and technical architecture) to use AlwaysOn features (SQLServer 2014).

A consultant told us to have (case of two servers to replicate): two servers (let's call them S1 and S2) in which we have two SQLServer instances (S1\DB1 plus S1\DB2 and S2\DB1 plus S2\DB2). 
S1\DB1 and S2\DB2 are masters and S1\DB2 and S2\DB1 are replicas.

With a schema:



Instead of basic architecture (4 servers, one SQL instance per server):


Consultant's point of view: less servers so simpler management and less costs. 

Question:is that a good way to implement SQLServer AlwaysOn?
Depending on your requirement, both solution are OK.
In first schema, imagine to have a fault on a server. Or you simply need to do updates. Can a single server handle your work load? If yes, you can save money.

You will need to handle correctly multiple IP for server, or different port for the istances, but it is not a problem if your department is skilled.

I mixed mutiple istances in multiple servers, also mixed failover cluster istances with Always ON. My only suggestion is to implement on SQL 2016 or 2017 if you can. It introduces DAG and is a bit more mature.