What pressure is created by the electrolysis of water?
The maximum gas pressure you can expect in a Hofmann Voltameter is the atmospheric pressure plus the pressure from the water column in the middle cylinder ($1~\pu{atm} / 10~\pu{m}$).

For a reasonably sized apparatus, you will not reach conditions where you had to consider "back pressure" effects that might stop production of hydrogen or oxygen.

Given that the cross section of all three cylinders is the same, the pressure of hydrogen is slightly higher than oxygen pressure since the water column that adds pressure is higher (as can be seen in the Wikipedia article you've linked.)