Subject to what Lori said, I'd recommend:

1. Water filter: Sawyer Squeeze (2-liter system, plus some extra bottles), with two additions. First, take along something to "scoop" water with; any small-mouth bottle will not fill easily from anything but falling water. I cut the top off a 1-liter Sawyer bottle, and it works fine. Second, go to a school supply store and buy a "tornado tube" (in the science area, about $3); it's a small piece of plastic that screws onto the filter outlet, and allows you to screw a small-mouth bottle to it.

2. Black Diamond Trail Ergo Cork poles.

3. Any of the silnylon tarps, in a 10x12 foot size, will be palatial for two.

4. Bivy: Integral Designs makes some good ones; the REI Minimalist is really good for the price, and really good for use with a tarp. However, for what you'd pay for a tarp and two bivies, you can buy a good two or three person tent (Big Agnes Fly Creek or Copper Spur series; MSR Hubba Hubba)) and carry less weight. I went from a tarp-and-bivy setup originally to save a couple of pounds over the then-available tents; I went back to a tent when lighter tents became available that weighed less than the tarp-and-bivy.