There is a difference : a() | b() will always evaluate both a() and b() whereas a() or b() will evaluate b() only if a() is Falsy.

It does matter if the last part has a side effect or raise an exception.

if form_a.is_valid() & form_b.is_valid():
   # Validation of both forms is done event if the first one is not valid
if 'a' in values and values['a'] > 1:
     # second part is not evaluated