I am pretty new to PDAs and I was solving a problem which asked to design a PDA for the following: $a^n b^{2n}$.
The transitions on the PDAs I've encountered so far have pushed only one symbol onto the stack at any time. Can we push two symbols? Eg: for every $a$ I encounter I push two symbols. Thus, the transition will be:
$\Delta(q_0, a,0;000) \to \Delta(q_0)$
where "$a,0;000$" indicates pushing two zeroes onto stack for every $a$ that it encounters.