$Pb(NO_3)_2 + 2NaCl$ → $Pb(Cl)_2 + 2NaNO_3$
If you have 4 moles of $NaCl$ & 4 moles of $Pb(NO_3)_2$
It takes 2 moles $NaCl$ to react with 1 mole of $Pb(NO_3)_2$ **AND ****we have the same amount.
$4molNaCl * \frac {1mol Pb(NO_3)_2}{2molNaCl} = 2mol Pb(NO_3)_2 <4mol$
2 mol is NEEDED to react but we have 4 mol so it isn’t the limiting reactant
$4molPb(NO_3)_2 * \frac {2molNaCl}{1molPb(NO_3)_2} = 8mol NaCl >4mol$
8 mol is NEEDED to react but we have 4 mol so it is the limiting reactant
∴ $NaCl$ is the limiting reactant
You DON’T have to do both, knowing one is enough, as seen through this quote by a wise man:
“If you flip a coin and land heads then you know the other side is tails”- Ethan Dusto
If you know that one reactant is not the limiting reactant then the other one must be the limiting reactant (only for reactions with 2 reactants)