Our model allowed us only to do INC, DEC and ZERO? on a counter, and it's easily seen that you cannot implement 2 counters using 1 counter with only ZERO?, INC and DEC, for instance by noting that you can recognize a^n b^n c^n language using automaton with two counters, you can (trivially) implement automaton with one counter using pushdown automaton, so if you were able to implement 2 counters using 1 counter, you could recognize a^n b^n c^n using pushdown automaton, which is easily seen to be impossible by applying pumping lemma for context free languages and usual characterization of pushdown automatons in terms of context free languages.