zacky, it doesn't look like anyone else is going to answer so I will have a go.

I don't think you can do what you propose, unless you can break the centre tap and reconnect both secondaries so that they are in-phase.  If you simply connect both outer ends together you will get no voltage output and a very large circulating short-circuit current in both secondary coils, because the voltages at either end of the secondaries are in anti-phase.

If you can disconnect the secondaries at the centre tap, then a 12V 6A output can be obtained by pairing the leads at the same ends of the secondary windings.  If you are unsure, you could experiment by connecting togther one lead from each secondary winding, and putting a current-limiting resistor (say, 1kohm 0.5W) between the other two leads.  If you get 12V output and near zero current in the resistor, you have paired the correct two leads, if you get approx. 0V out and a resistor current, swap the leads on one of the secondaries.  Hope that makes sense.

Whenever you parallel up active circuits there is always the possibility of a circulating current due to a slight mis-match but I wouldn't think it will be a problem.  For reassurance, you could always measure it with an ammeter.

UKpete,thanks for you help, regarding to the slight mis-match and the possibility of a circulating current, that could be a problem at no load , but I think ,once we have a load ,one coil will supply current slightly more than the other into the load and due to its coil resistance and the voltage drop across it we finally will have equal voltage across the two winding terminals and will have no circulating current - I think