This past week has been a water well learning curve for me. A couple of weeks ago our well started pumping some air along with the water. This past weekend we ran out of water. The well has been in operation since last August with no troubles whatsoever until now. Total drilled depth is 330 feet; (4 inch pvc shaft), with the 2 hp pump located about 100 feet down. We have not had much rain and the rice farmers have been pumping non-stop for weeks, so evidently the water table dropped enough to not allow the water to come up as far in the well shaft, so the pump started getting air. The pump also overheated enough to come off the end of the 1 ¼ inch plastic pvc supply pipe along with the 4 inch shaft slightly collapsing at that point.
The well guy came out and got the pump out and will replace it with another 2 hp pump, but slightly smaller in diameter so it will fit into the shaft without restrictions. He was also going to install some type of gizmo to prevent the well shaft itself from being drawn down again. The new pump will be at 250 foot depth to ensure an adequate water supply.
The nice thing is that he is doing all this at no cost, although I said I would pay him for the additional piping needed to increase the depth. He said that he should have had the pump lower in the first place, but at the time, the 100 foot depth was more than adequate.

Anyway, after being without water for several days, I've really been thinking of how to have an additional water storage tank (say 500 gal) in case the pump ever fails again.
What I want to do is have a tank that would be plumbed into the house water supply line so that the water to the house would go through the spare tank first to ensure that it had a constant fresh supply. The piping routing would be: pump to pressure tank to spare tank to house. I would also have a small auxiliary electric pump that could be fed by either house or generator power so that I could maintain water pressure in the house plumbing. Some bypass plumbing and valving would also be needed to avoid pumping water backwards to the pressure tank, and also to isolate the extra pump during normal operation. I assume that the spare tank will need to sealed to withstand the normal water operating pressure.

The main routing would look like this (spare pump/valves not shown):

Well Pump ---->Press. Tank----->Spare Tank---->House

Sorry for the very long post, but you are my best source for info. Does this idea look feasible?