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:
The same procedure is followed when Clerics or Druidic Priests wish to move from one level to the next. The lower level career must have been fully completed, with all available skills and advances having been taken (plus a minimum of two spells), and the character must spend 100 EPs per level to gain one roll on the Advance Table.
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.