You can remove the IF portion of the rule since the AND returns a True False which is what you want back from your validation rule.
When checking if the date is empty using LENGTH your EQNUM should be to 0 not a String "NULL"
AND
EQNUM
LENGTH
DateField A
0
EQNUM
LENGTH
DateField B
0
Your second rule
AND
EQNUM
LENGTH
DateField A
0
NOT
EQNUM
LENGTH
DateField B
0
--Bill