My car, which has a 12v silver calcium battery, is flat and I would like to get on the road as soon as possible. I got two car battery chargers that are not identical, but both can operate at 6 an 12 volts. Is it possible (and safe) to charge the car battery using the two chargers at the same time?
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.
Silver calcium batteries (just like regular lead acid batteries) need a constant voltage charge regime. The charger must be able to supply sufficient current and this goes without saying or it is unsuitable.
So, if you have a charger that provides the 14.4 to 14.8 volts and has the required current delivery for the battery then what does another charger bring to the party?