Also, the consul Publius Laverius Laevinius is thought to have had four legions with accompanying allies at Heraclea in 280BC, so was there a specific law limiting a consul's command to two legions, or are you talking about the customary allocation? Of course, at Cannae there were 8 legions plus allies, so they could clearly change things when they needed to!
At Zama, Hannibal did not use a tightly packed infantry, deploying three massive lines instead. Again I quote Livy. “Hannibal put his elephants (80 of them) right in the van of his army; behind them were the Ligurian and Gallic auxiliaries with a certain proportion of troops from Mauretania and the Balearics. In the second line he stationed his Carthaginian and African troops together with the one legion from Macedonia; Then a moderate distance to the rear of these, came a reserve of Italians and Bruttians.