nita, as a reference, keep in mind that the first pic shows an 8'x10' tarp over a two-person tent, and it's just barely covering it. For a tarp to cover two tents in the second configuration, you'd probably have to go more than double that size. At that stage, I take it to the next level, and build an emt (electrical conduit) frame. However, if you want to keep it as simple as possible, here's how I'd go:
Large tarps are cumbersome to work with. So, instead of one large tarp, I'd probably go with two moderately sized tarps, each sized around 8'x12' or thereabouts, with the longer end folded to make the wall and roof. Don't go with the cheap blue tarps. Get the heavier duty silver tarps or shade net, and make sure they have grommets on all sides. I would use six poles instead of four. You can overlap the two tarps at one edge, giving you a double grommeted "middle seam" that you can mount on the two center poles.
Now the key to making shelters like these strong instead of "flimsy" is guylines, so bring plenty of rope and stakes (these take up very little space in your car). If you set it up as in the sketch, but it doesn't seem strong enough, you can double up on the guylines, especially at the corners, with one line running forward and one line running sideways from a single pole. Obviously, you will also stake the bottom edge of the "wall" to the ground, and you can use as many stakes there as you feel is necessary. Also, learn a good knot if you don't know one already. The taut-line hitch I linked to in the first post is my favorite because it is a slipknot (length-adjustable) so you can really tighten it down. It just takes a little practice, which you can do in your leisure time in front of the TV or whatever.
Note, I have just sketched this up, but have never pitched this setup. It should work, but as Bob said, I would get the materials asap and start experimenting in your backyard or what have you. It would probably be easiest to have at least two people setting it up.
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