i've never used crystal, but i believe it uses SQL, right?
try the COALESCE function, which returns the first non-null parameter value reading from left to right
so COALESCE(field1,field2) will return field1 if field1 is not null, and field2 if field1 is null
and if both are null, then of course it returns null, unless you add a default value as a 3rd parameter --
COALESCE(field1,field2,'booyah')