Toilets, Sinks, Showers, Tubs and Disposals - Both Toilets Not flushing well

We have two toilets in our home which sit back to back. We have a septic system. One never flushed well and the other was fine until recently. Now they both have to be flushed repeatedly. What do you think the problem is?

Could be a number of things.
Start the process of elimination, by reaming out any mineral build-up in the holes under the rim of the bowl with stiff wire.
Check the flappers to see that they're staying up long enough to allow the full amount of water into the bowl when flushed. Replace the flappers, if necessary. Shorten the chain of the new flappers until it just allows the flappers to seal completely.
Almost anything can get hung up in the built-in trap of a toilet (plastic toys, toothbrushes, etc.). Clear the traps with a crank-handle toilet auger. If the problem persists, you may have to pull the toilets to auger them from underneath, too. (Replace the wax rings.) Snake the closet bends while you have the toilets up.
If the above doesn't help, open up a clean-out plug and snake the drain line from the clean-out to the septic tank with a heavy-duty plumber's snake. Check the top of the stack on the roof for clogging (leaves, debris, etc.), and flush the stack down with a garden hose sprayer.
Good Luck!
Mike

Missed one item Mike....Ensure you have the proper water level in the tank prior to flushing.

I would also like to ask: Do you use the drop in toilet tank cleaner disks? Sometimes these get down to a small enough size to get sucked into the jets of the toilet bowl and cause problems.