This is a common request, but so far the product doesn’t support a custom multi-select reference to object. At Kolme Group, the way we look at relations between objects is that there are three kinds:

1. 1-to-1: This is a regular Reference to Object (RTO) field. One object can reference one other, although the other object isn’t “aware” that it is being referenced.
2. 1-to-many: This is a Reverse RTO. One record can be referenced by many records, and can display a panel of them. But there’s a limitation: If Project A is referenced by a reverse RTO field on the user record (for example), then each user can only reference one project. If Tom, Dick, and Sally reference Project A, then none of them could ever reference Project B without being ripped off of Project A.
3. Many-to-Many: This is the scenario that’s empowered by Link records, and this is how Resources on a  Project work. Tom, Dick, and Sally can all be Resources on Project A, AND they can be Resources on many other projects as well. 

So in your use scenario, you probably need a Link. The problem is that you can only create custom Links when one of the objects is a Custom Object. For links between two out-of-the-box objects, only the preexisting standard links can be used to relate objects.

Thanks I appreciate this, I suspected it was the case but nobody was saying it outright so I was starting to waste time on a wild goose chase 

I have created a workflow rule that updates a text field each time Resource is updated, and this meets my reporting needs for a secondary resources field. Code below to support others with similar goals. (resource has been restricted to one at a time, below code check if the new resource has already been added, if yes code is the same, if no display name is added to the existing text.

If(InStr($C_Secondaryresource,$C_AssignedResourceNotifications.DisplayName,FALSE)>0,

   $C_Secondaryresource,

   $C_Secondaryresource +" " + $C_AssignedResourceNotifications.DisplayName + ",")