You are still trying to apply magic to the system – not sound physics. You are not just heating 60 or so gallons of water, which would take an hour to heat up with the electric element. The pex tubing is connected to the concrete slab (or it wouldn’t be considered a heating system, correct?), so the mass of the slab is equivalent in mass to another 600 gallons of water. This would take another three hours to heat up from 50 to 70F – IF the slab wassn’t losing heat from its surface at the same time.
The other defiance is that the PEX company wasn’t consulted as they would tell you not to make the length of a run more than 300 feet. The 800 feet should have been divided into three shorter circuits so an ordinary heating circulator can handle the flow.
Two expansion tanks in different locations with two circulators will confuse the pumping rates of the circulators and probably result in repeated pump failures.
According to the Bell&Gossett System Syzer, the friction loss per 100 feet of your tubing is 7 feet per 100 feet which is normal work for one circulator. You have 8 times that – or more than 56 feet of head; no circulator is made to overcome that pressure. It is a GOOD thing to know physics.
Go to the end of the slab where you know the circuit loops are and break out two loop ends approximately where the loops would be equal to making three loops and repipe.
Go to http://www.hydronicnetwork.net and read the free lesons on hydronic design so you can understand the process of developing a system without these errors.