Totals in linked subforms

I have a main form with 2 linked subforms, much the same as the customer
order form in the Northwind database.

The main form contains details of a site. The subforms relate to houses on
the site and the funding of them.

I can easily create a subtotal for the first subform (which is total cost
for all the houses). So far I have only been able to create a subtotal (total
funding) for the individual block of houses on the second subform. This is
because when you click housingID on the first subform, the second subform
only shows the relevant funding records (you will see what I mean if you look
at the customer order form in Northwind).

Somewhere on my main form, I'd like to display the total funding for all the
houses on the site, not just the various filtered lines of the second
subform. Perhaps this is impossible or perhaps it is not the right way to do
things, but I thought I would ask anyway!

If it is impossible, any advice on alternative stategies would be much
appreciated. Thank you.