In regards to your question,"Why two cleanouts?" unless your plumber has a few gray hairs it is highly unlikely they have ever seen that before.

What you have is a "whole house trap" With some extremely rare exceptions as a rule whole house traps were outlawed in the late 60's to early 70's.

A whole house trap is an inverted U section of cast iron with a cleanout on the top of each of the risers of the U, then a 3" line is tee'd into the riser just below the top on each side. The whole house trap was installed at the junction of the "house sewer" (line from the municipal sewer or septic tank to the house) and the "House Main Drain" (The main sanitary line inside the structure). For structures on slab the junction is usually made approximately 3' outside the footer wall while in structures that have a basement or crawlspace it may occur immediately inside the footer wall. Under the present codes we are required to install a cleanout at this point but whole house traps are prohibited.

The purpose of the two cleanout caps is because a drain cleaning machine cannot pass through a whole house trap so it had one access to rod the house main drain and the opposite access to rod the house sewer line. Genarally today we install a "Rodding Tee" which is a Tee with a radius curve in both directions to permit using a drain cleaning machine. (A rodding Tee is the only Tee that is permitted to be installed on a horizontal line). In some jurisdictions today they require two Wye & 1/8 bends be installed, one in each direction to permit rodding the line both ways.

To understand the purpose of the whole house trap we must examine a bit of plumbing history. Prior to world war II it was common practice for a municipality to discharge raw sewage in a local river, bay or estuary. In those days storm drainage and sanitary waste was conveyed by one common sewer pipe thus the municipal sewer was vented through the curb side storm grates.

shortly after WWII it became a common practice for cities to install sanitary waste treatment plants to treat the sewage before discharging the excess water into the rivers. The capacity of the sewage treatment plants was determined by the average volume of sanitary waste generated by the community at large. It was soon learned that the shear volume of storm runoff overwhelmed the treatment plants and resulted in mass spills of sanitary waste into the waterways. In order to prevent future spills the cities then installed a second set of sewer lines dedicated to storm drainage. Understanding that storm runoff is pure water it was then determined that the storm runoff could still discharge directly into the watershed. The problem that occurred was venting the sanitary sewers. Previously the sewer gasses vented out through the storm grates but now there were no storm grates on the sanitary sewer and each house had a whole house trap so the gasses could not vent through the house vents. This resulted in extremely high levels of sewer gas in the sanitary sewer lines which commonly reached explosive levels. (In one community in southern Illinois in the mid 50's a sewer exploded ripping a ten foot wide trench for 10 or 12 blocks down the main street of the town, killing a number of people and setting numerous structure fires.)

The solution to the venting problem was to eliminate the Whole house traps. Once they were removed as each new structure was added onto the sewer main the vents in the structure provided a proportionate amount of additional venting for the main. In this manner instead of having the sewer gasses concentrated at one point they are evenly distributed across the entire community where they normally will not reach toxic or explosive levels at any one point.

Occassionally we still find a whole house trap in older homes such as yours. In most jurisdictions there is no mandate to remove it but if it should happen to fail for any reason it should be removed and replaced with a straight section of line with a rodding cleanout Tee.

Now in regards to the other cleanouts in your home. In addition to the main cleanout at the junction of the house sewer and house main drain the code requires a cleanout at the base of the main vent, on the upstream end of all branch lines and at each point where a line makes a change of direction greater than 45 dgrees.

Now for your question about the floor drains. In many homes that have a basement the main drain exits the structure at an elevation higher than the basement floor. In this situation a floor drain is below the sanitary drain so it is generally routed to the sump pit or to a storm drain.

thanks lazypup it all makes sense now.
fyi: the plumber I ended up hiring (young guy) said "it might go to the storm pipe or it might not". He told me we would see when he busted the floor out and gave me a flat price to "do whatever needed to be done to make the drain work again"