A dual projector setup is difficult to align properly. Generally requiring several hours, and then you have issues with color accuracy not being the same.
Depending on screen size, you really shouldn't need two projectors, but a single good quality projection setup can likely give you a 200" or so diagonal after dark.
My BenQ W1070 gives me a 161" diagonal in a basement (no windows) in lower power mode with no issues at all. I would expect it could handle 200" of diagonal just fine as well. Also, there are lower resolution, but higher brightness LCD models which aren't terribly expensive that are worth consideration.
There is a lot of information on outdoor projection at http://www.backyardtheater.com
Worth taking a look there as well.
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