100 gallon propane tank backup?

I'm going to guess you meant to write 1000 gallons and 100 lbs, not 100 gallons and 100 lbs.

The "standard twin-tank approach" used for 200lb and smaller tanks where two tanks are used has a special valve that uses gas from one tank until it runs out, then switches to the other tank and raises a (usually red) signal that the "first tank" is empty. When the empty tank is replaced, a lever on the valve is switched to make the tank that was second, first, the signal retracts, and the new tank becomes the reserve until the next tank switch.

For 250 gallons and up (typically 250, 500, and 1000 - after 1000, multiple tanks again) A tank gauge is used and when the level is at or below 30% you call for a "non-emergency" refill if your supplier does not already have you on a regular delivery schedule that keeps the level from getting that low. If you happen to notice that it's down below 10% you call for an "emergency" refill because it's cheaper to pay extra for that than to have the lines purged when they get to refilling you after you run the tank all the way out, and also because "you can't run out."

Unless your use is highly variable (in which case you should check frequently to know where the tank level is) or your tank is woefully undersized (in which case you should get a second full size tank, or a larger tank) it's absurdly rare to "run out" on either a scheduled refill or a "call at 30%" plan.

So, what sort of reserve could you get if you could get a 100LB tank as a backup? 42 gallons, more or less. i.e. less than 5% of the 1000 gallon tank you likely meant, because "100 gallons" is not a typical tank size for propane delivery.