Yes and no.
PCIe has a concept of both lanes and ports. So the Southbridge has those 16 PCIe lanes configured to a single port. When you see a board that has 2 slots and it can be 1x PCIe16x or 2 PCIe8x it means the PCIe switch inside the southbridge is capable of dynamically configuring the port to lane assignments.
Just splitting the lanes physically will do no good. The PCIe switch inside the southbridge won't "know" there should be 3 or 7 more logical ports.
So what you need is a PCIE switch (a chip which can route PCIe lanes to ports) and that would require a custom PCB.
There are existing solutions which provide PCIe expansion but they are way to expensive. Here is one example:
http://www.magma.com/expressbox16basic.aspIt adds 16 PCIe slots in an external chasis from a single 16x port. So hypothetically take a MB w/ 4 PCIe 16x slots connect each one to a 16 bay expansion chasis and gain 64 slots for GPUs.
Two problems. One the chassis and expander is an insane $4500. Two AMD stupidly limits you to 8 GPU.
Given the 8 GPU limit I have found the easiest, fastest, and most efficient setup is a motherboard with 3x 16x slots spaced two spaces apart and use 3x 5970s for 6 total GPU. No other method seems to come close to that in terms of cost.
