it osunds like a normalisation issue. an interactive form can only deal with adding to a single table at a time (without code)
if you are ending up, as you say you are - with a form that needs autonumbers (or even just PKs) from two separate tables, this points to a many to many relationship, or a normalisation issue (or both)
you need ot review the design of the dbs, and then the functionality should flow properly.
can you post your table structure, and what you are trying to do.
Notye in general however, that access is NOT the same as excel, and the table structure will look rather different. its more like having many small spreadsheets, rather than one big one.