Can I specify this (property: interest) for each user on a single KNOWS relationship or would I need to create two relationships between the users and set the attribute on each of the relationships?
You will need two relationships.
You could do it with one but then you have to keep two properties in the relationship and information about which property goes with which node. Much easier with two relationships.
From comment:
Can I keep them as bi-directional or would I need to use directional
in this case?
Relationships are always directional. It is only when you query that the concept of bi-directional appears, but that is not really bi-directional, it is without direction, e.g. (a)-[r]-(b). So you would use (a)-[r]->(b) and (b)-[r]->(a) or (a)<-[r]-(b). If you query with the direction, then you know how to apply the relationship property.
I typically do more of my work with Java as an embedded application instead of Cypher and it pays to use directional queries as it makes for less code to do the associations.
Note
Since your case is so simple, just try various methods and see what works. Remember to keep track of how long the quires take and if necessary add indexes. Also use the query profiling tool to make sure you are making effective queries.
