I would run the propane lines under a concrete drive, if I thought I could afford the concrete /forums/images/graemlins/grin.gif. Seriously, there are some service stations locally which have undergorund tanks feeding a propane dispensing station, and all of the plumbing (but not the tank itself) is under concrete. When we were fabricating outdoor kitchens, we often plumbed a gas line in under the concrete patio and up where the grill was to be. There's always the slight risk that a cosmic upheaval will thrust a rock against the polyethene line at exactly the wrong place, but I;d take the risk. Of course, we have no rocks -- or, at least, no rocks larger than a grain of sand /forums/images/graemlins/crazy.gif.
Personally, I would worry about running the generator for a long period of time and put the larger tank there. That's because I would have sincere doubts about the ability of the gas company to make timely visits to refill during a general emergency. In normal times, they will be able to keep the smaller tank full for your normal use.
The plumbing to switch from one tank to another could be done, but it would involve a lot of designing, a lot of valves, and possibly some confusion, along with expense. The easiest way to do it might be to plumb both tanks into a manifold, then withdraw different circuits from the manifold. There would be valves allowing either or both tanks to flow into the manifold, and valves to allow either or both output circuits (house and generator) to withdraw from the manifold. Of course, there would be no simple way to determine usage by each of the circuits without metering the output lines. Otherwise, the combination of valves and bypasses would start to resemble a railroad switching yard.
As for buying the tanks, I'm still hoping for a deal. The state LP Gas safety inspector for our region of Florida is a good friend who lives just a few miles away in Okeechobee. He's doing a low-key search for me; when I get closer to the actual installation he'll step up the search for a gas company who wants to get rid of an excess unit. What I really want to find is a dispensing system so I can fill my own RV and portable tanks, but I don't want to have to pay what that's worth. /forums/images/graemlins/ooo.gif I know it's possible, because my friend has one...but then, he has a lot of clout.