How to charge an AGM battery fast?
If you connect two batteries in series for charging, they MUST have the same state of charge  - ideally, they would have been used in series, as a 24 volt battery.  They should also be same make, model, and vintage/use history.

How long it will take to charge the batteries will depend on their state of charge.  In theory, if they were fully discharged, and the charger could maintain a 50 amp charge current until they were fully charged, they would be fully charged in a little over four hours.

A smart charger will deliver its maximum current until the battery voltage under charge reaches about 28.8 volts (or so)(for a 24 volt battery), and will then reduce the current, maintaining 28.8 volts, then reduce the voltage to a float charge level.

See Batttery University for more details.  (AGM batteries are a variation of lead-acid, which that page discusses.)