How can one charger try and charge the other one? If that would be true then one charger would try and charge itself also.
In simple terms, chargers send electricity to something that "has less" than they do--in other words, when a charger has an "excess" of electricity and a discharged battery, or a device, "needs" electricity, then the charger sends its "excess" to the "needy" device.
If you only use one charger, and it attempts to charge a discharged battery, or to power/charge a device, the charger will always be in the situation of "having an excess" when compared to the discharged battery or device.
But, if you connect two chargers together without proper isolation, there will be (many) moments (in the charging cycle) where one charger thinks that it has an "excess" when compared to the other charger, and will try to send its own excess to the other charger.
This is the problem.
In case it helps explain a little more, the charging cycles are patterned after a sine-wave, not a constant-line. There will be times when one charger's cycle is not the same as the other charger's cycle, and if they are not isolated, they will try to "charge" the other charger.
Does this make things any clearer?
EDIT: All of my notes above are discussing two identical chargers. If the chargers are not identical, then the problem gets worse.
Hope this helps,