Okay, so I want to make sure that I follow this. We have a family of four (Two adults, Two children, ages 7 and 2). So, instead of only getting fast passes for three rides, we can get six fast passes with rider swaps (at the ride entrance) as long as our DD2 isn't able to ride? This will be our first time using either FP+ of RS.
In your particular situation, its kind of complicated, because the 2yo can't get a Fastpass+ reservation (they would not have a ticket, and wouldn't need one anyways to join an adult who does)
The concept is this - first, basically how Rider Switch will work...
Say you want to ride both Soarin' and Test Track, but your 2yo is not tall enough for either. Fastpass+ allows you to only choose one of them due to tier limitations. You choose FP+ for Soarin', and ride Test Track standby.
Since your 2yo is not tall enough, you go to the cast member at the entrance of Soarin', present the 2yo for measurement (if they ask), and ask for a Rider Switch pass. One of you stays with the 2yo, while the other two use their FP+ to enter. When they come back, you then enter through the Fastpass line with the RS pass, and your 7yo can join you. Bonus for the 7yo - he gets to ride this twice.
At Test Track, same thing, except the first two have to go through the standby line, so you have to wait longer.
Now, for this "loophole"...
Instead of getting FP+ for all three of you for Soarin', you switch _one_ of them to Test Track (whoever will stay with the 2yo first at Soarin'). Be sure to pick a later time for Test Track. Then go to Soarin' and do as above.
When it is time for the FP+ for Test Track, you again ask for a Rider Switch pass, and the adult who stayed with the child at Soarin' ONLY enters via the Fastpass line (you only have one FP+ here...). When they return, then the other and the 7yo can ride using the Rider Switch pass. The advantage is that you didn't have to wait in the standby line at all. The disadvantage is that the 7yo has to wait this time.
If the 2yo was 3yo with a ticket and thus able to get FP+ as well but still too short to ride, then you could split the FP+ two-and-two, and one of you would have to use the 2yo's MagicBand for one of the rides.
Because of the complexities in doing this, and having to swap around MagicBands, etc. (and perhaps the fear of getting "caught" - the CMs can see limited information on who scans the MagicBands - even if the CMs don't particularly care of people are swapping them around) will likely limit the number of people who would bother, which already between needing the child to get the Rider Switch pass and even knowing it is possible to do this is probably a small number.