My suggestion would include making sure that there is a central low point in the design, or perhaps use a drilled pattern. I have a plastic "splash" guard that runs from the under the front cladding all the way back to the forward end of the transfer case. When water, dirt, mud get in there, it stays because there is no where for it to go, so if you put a crease down the center having it come down toward the ground, that would help in drainage. My other concern would be heat. If you do mostly rock crawling type off road, then it is even more important since there isn't a lot of air flow generated by forward motion. Maybe holes in the bottom, like the prerunner skid plates?
As far as two pieces go, a solid frame rail design with removable under pieces and thinner cross members, good idea, I like teflon. You can get it in sheets, its lightweight, very convenient if you get on rocks and stuff since most of the time you would slide over just about any obstacle, and if you get gouges, you can heat it up with a torch or nice heat gun and smooth them back out again (not too hot, it will liquify or burn). There would be no galvanic corrosion, or any other type of corrosion, you could inset the mounting bolts to minimize risk of breaking the head off of one as well. I put teflon runners down my boat trailer boards, my boat came off the trailer like a greased monkey after that. It was one of my better projects and was cheap as well. My boat was heavy, about 5.5Klbs geared up, a Mako 191 center console with solid t-tops, twin Honda 4-stroke outboards, setup to troll with 7 Penn international 50s...I miss my baby.
Sorry, I was just flashing back...
But either way, keep us posted on your skid plate project, I am waiting to see how Riff Raff's deal works out, if not, I gotta do something before Moab and it may just be ghetto fabulous pending more time and money.