Set your charged voltage to 14.3V (although this is not the issue here I believe) and try a tail current of 1%, I found that OK with high quality AGM.
The second issue of the chargers going into float early can occur if you have too much voltage drop in your cables from the chargers to the batteries so the chargers see a higher voltage. If the voltage drop is between the shunt and the battery that can affect synchronisation to 100%.
Finally, does your engine charge the batteries by an alternator and if it does do this, does the BMV register the amps from the alternator. If the alternator negative does not flow through the shunt then this can result in early jump to 100%. See another FAQ on shunt wiring if it does not get registered.