List validation formula problems ... can't find the right syntax.

Help,

I am using Sharepoint 2010 and am trying to add a list validation that chcks to see if a field has a value, and if so, makes sure another field also has a value.  It equates to a conditional mandatory field.  So, say, if Field1 has a value, Field2 must also have a value.  Individually, I think I have each one working:

Field1 validation = LEN(Field1)>0 - this returns true if there is a value in the field.

Field2 validation = [Field2]<>"" - this states that Field2 must have a value.

I now want to combine them both somehow so that the formula will return false if Field1 has a value and Field2 has no value.  I have tried a variety of things but I usually get formula syntax errors.

Any thoughts?

Thanks - Peter