Do i need two submersable pumps for my pond w/ skimmer?

You don't really even need a skimmer at all. Read my articles at a href="http://www.squidoo.com/pondlady" rel="nofollow"http://www.squidoo.com/pondlady/a for all the reason why you don't. But even if you decide on having one, you only need one pump. From the skimmer, the hose goes over the waterfall. The pump would go in the skimmer, not in the bottom of the pond. The 4200 gph pump is a fine size for what you want.