Your devices are poorly calibrated, such is life with the cheaper end of the market. As a result they are targetting different battery voltages. When the voltage rises above what 2nd one is aiming for the current from it will drop to near nothing.
Your battery is at float voltage. If you discharge your battery so that the chargers are in bulk stage charging you will likely find that both devices are supplying maximum current. If you still see unexplained low current under bulk charge conditions I would recheck the voltages at this time and report the readings in this thread for further advice.
If you can adjust your controller's configurations by 0.1V steps you can possibly get them to share the load more equally when in float mode but it's not really necessary.
If your battery voltage rises above 27.6V, as read by your multimeter, in float stage charging you should reduce the voltage settings on the 1st charger as it appears to be reading low so it is trying to push the battery voltage up.
