If I understand correctly:

You have two tables which are connected via key, you want to place a grid for (one) or both of them on the form and have them stay in 'lock-step' as you move through the table...

I have done this before with two and even three related tables by setting the Relations and having each grids' Rowsource point to each table. Result is that each grid is populated with appropriate child record(s) as you move the parent table(s).

HTH,