But I cannot select a field on the first Subform to filter the second
subform on...
Typically you would not do this with a Filter; if you set the Master Link
Field and Child Link Field properties of the sub-subform correctly (the Master
being the name of a field on the subform) you should get the same effect.
However you CAN indeed refer to a field on the first subform to filter the
second, if you wish to do so. What specific problem are you having? Are you
putting the second subform *onto the first subform* as a sub-subform, or
trying to put it beside the first subform (on the main form)? Even that can be
done if you wish...