....
this.AddObject('MyCustomSession')
....


On VFP any object is a object tree with a root.
A object tree can not live into two dataSession.

With AddObject the Session is a object component of the tree object
where this is a node object.
Then MyCustomSession not build a private datasession.

For do this, create MyCustomSession into a private object tree.

On this case:

this.AddProperty('MyCustomSession',CREATEOBJECT('MyCustomSession'))

Fabio