Charging a car battery with two chargers simultaneously
This is actually hard to give any answer without knowing what exactly inside of your chargers.

But I'd say that any charger should have a diode on its positive output (cathode to output) so it should be safe. However it is pointless to connect two chargers simultaneously for the same (diode) reason (the charger giving lower voltage will not open its output diode and there will be no current at all).

I'd suggest to pick only one charger which is more powerful (basing on its description or on the chargers amperemeter). This is safe and 99% fastest way.