Though is there a relation between B and A which isn't the one used for the hierarchy?
No, there is only the 1:1 relationship between the two tables.
If not, it's not possible to add both to the typedlist for obvious reasons.
Maybe I'm misunderstanding how the typedLists work. In my case I'm working with a base class "Transaction" which has a subtype "LeadPurchaseTransaction". I want the typedList to include fields from both tables. If I add "LeadPurchaseTransaction", will I automatically be able to use fields from the "Transaction" table, or do I have to include a relation to that manually?