The most important Normal Forms (1NF-6NF,BCNF and derivatives) don't allow nulls in tables because all of them are based only on relations with values, not nulls. More usefully there is a design principle called the Principle of Orthogonal Design which specifies that tuples with the same attributes should not be permitted to appear in multiple places in your schema. It seems likely that your two transaction tables and having comments in multiple places would break that rule of Orthogonal Design.
You could create a parent transaction table that combines the common attributes of your two transaction tables, including the comment attribute (supertype/subtype pattern).