After you obtain the national dex, you can talk to Mr. Backlot and he will tell you a Pokemon that can be found in the garden that day. To get the Pokemon that you want in the garden you can save in front of him before you talk to him and if you dont get the Pokemon you want you can soft reset.
You cannot get two Pokemon in the garden at the same time (such as Eevee and Ditto) unless you get Eevee on one day and Ditto on the next (or vice versa) because the Pokemon from the previous day remains in the garden.