Do clerics have the same requirement to know two levels of a previous level before being allowed the next level, like magicians do (as listed under the Cast Spell skill on page 140 of the core rulebooks)?
Short Answer: Yes.
Long Answer: In Section 4 Priest Careers, on p. 149, Career Progression states:
However, as a multi-year GM of WFRP 1e in '87-'88, I can't think of ever enforcing this rule. Mind you, I think the rule has an element of strong flavor to the 1e game that could spice character development, but it also feels forced and mechanical (for clerics and arcane).
The rule is not present in WFRP 2e.