You can enable Redirection in the settings of each individual product and provide a URL where we would direct the buyer after that product is purchased. To specify a redirection page for 1+2, I think it would be best to create a third product which you understand to be a "bundle" of 1+2 together, then you can give that product its own Redirection URL:
To make this happen as automatically as possible, do not customize your E-junkie thank-you pages -- i.e., put nothing in the Message (HTML) field of each product's settings, and nothing in Admin > Account Preferences > Common Thank-you Page HTML field -- and use Buy Now buttons (rather than cart buttons) to send buyers to instant checkout for just one item at a time.
Note that different checkout methods vary as to how they handle the post-checkout transition; some can auto-redirect the buyer, others only present a link the buyer must choose to click to proceed. For checkouts that only present a click-through link, if the buyer does not choose to click the link, we can still send a product-specific thank-you email with a link that would lead to the Redirection URL. For this case, you would probably want to delete your Common Thank-you Email in Seller Admin > Account Preferences.