It sounds like there is no disadvantage to having two 18"x24" sump basins. Am I correct?
I would like use two sump pumps because I am pumping so close to the basement floor. Either 1/2 hp pump would be atequate to keep the basement dry and the duplication would give me extra protection against flooding (i.e. both pumps would have to fail for the basement to flood).
Is there a prefered way to stager the pumps?
You can have a pump in the first pit as well. Just note the pipe connecting the two pits is located at the bottom of the pits. This way they both fill up equally. I would install both pumps at the same height, and use the alternator I posted. This way both pump would get the work out every other turn on. and if the water gets two high and activates the high level switch it will sound an alarm and energize the other pump.
OK if the pump is sitting on the bottom and you where getting the 10 seconds on and 10 seconds off that is an issue of an undersized basin. Raising the pump higher into the pit is where the water has to get over the drain tiles to turn the pump on is using the drain tiles as a holding area which is why it takes 10 minutes to clear out the water.
Raising the pit above the floor so you can make the turn on level even higher than it is now would be a mistake. IF the after gets any higher your drain tiles are already full and with the higher turn on point you will start to notice seepage in the low spots of your basement. To properly solve your problem is a larger diameter pump pit, do not worry about it being deeper, that will not hurt at all. I would install a 36" by 36" basin. This way you can install both your ½ hp pumps and use an alternator. This way each pump gets to run every other time. The alternator also has a high level float that when activated will apply power to the other pump. Below is a picture of this alternator.
The way this unit works is you plug in both your automatic pumps with their switches intact. The alternator will automagically turn on one pump then the other on each pump cycle. The switch pictured is a high water alarm switch in case a pump fails it will sound an alarm and energize the other pump.