Difference warehouse shipping

My case is:
I have 2 supplier, i.e. supplier A and supplier B in difference place,
They send product for me. but i offer free shipping if customer buy more than 100.00,
so if customer choose product from supplier A + supplier B total cost of 100.00, they get free shipping.
problem is, I need to pay the both supplier for the shipping, than I will get loss on shipping fees (pay double).
Can any body give me a suggestion to solve this issue?
for example, to tell the system there is two warehouse so that it will give free shipping either one get 100.00.
i.e. product from supplier A cost 100.00 = free
product from supplier B cost 100.00 = free
product from supplier A plus supplier B cost 100.00 = not free
Hope you understand what I'm saying

I saw there is a location field on product => data, can anybody tell me what is this field for.

thanks.

You could use Product-Based Shipping to do this. You would set up two rates for ONLY ANY of Supplier A's products and ONLY ANY of Supplier B's products, then a default rate for your normal cost. If you set the Sort Orders to the same value, and the Multi Rate Calculation setting to "Lowest", it would then give Free Shipping when ONLY ANY of A or B is in the cart.