Yeah why only two cards per MOBO, you can at least put 4
I can, but when I put everything in an equation, it costed me the same as getting motherboards with 2 cards. So I went for simpler solution 
2 extenders cost around 5$ and I doubt a second MB, CPU, RAM, HDD cost less.
poor NOOB, you don't need expensive MBs having 4-6 PCIe 16x slots, you can just use the 1x slots you have free on the MBs allready you have. You don't have to use a powerful PSU just bridge two of the once you bought.
And there are simple cheap MBs with 1 PCIe 16x and 6 PCIe 1x slots like ASROCK H61DEL it is made for mining and u can put 6 cards on it.
That is what I do - 3 machines with 18 cards total - http://www.youtube.com/watch?v=OCTDN05LKao