Here is how I got the achievement with exactly 10 species…
Start a new garden and remove all vegetation and water to keep from attracting more piñatas.
Have the complete Tower of Sour to keep out Sours.
Purchase the Captain's Cutlass and build a wall to keep out the Ruffians and Prof. Pester.
You need 6 of the same piñata and NO OTHER TYPES OF PIÑATAS in your garden. It doesn't matter how you get them. Gretchen fetched most of them for me. To repeat, the 6 piñata in your garden must all be the piñata you are working on for this challenge. I tried romancing two types of piñata simultaneously and neither worked until I sold everything else and had exactly 6 of the piñata I was working on.
In order to keep the garden clear of other piñatas, I only used ones that eat fruit or flowers for their romance requirements. Remove the flowers and seeds immediately after their use to keep from attracting other piñatas. I used...
whirlm, sweetle, bispotti, bunnycomb, mothdrop, mousemallow, buzzlegum, taffly, flutterscotch, raisant.
Might as well start with Whirlms since they are the only thing that will still be attracted to your barren, lifeless wasteland of a garden.
In the romancing mini-game, you will know you have the right conditions when the pink hearts in the maze are blinking. You will need to collect exactly as many hearts as are shown in the lower left side of the screen, and it will always be 5 hearts. At the end of the game, it will say "twin bonus", but only if you collect the 5 hearts and reach the other piñata with at least 10 seconds to spare. If you finish the game with less than 10 seconds left, you will not get the all important "twin bonus" message. Try again.
Whack everything that comes into the garden but isn't a piñata that you need for the challenge.
This plan might sound expensive, but when you sell all of the piñatas, houses, etc., you usually break even or maybe ahead a little. Also, the piñatas often hit maximum candiosity. While you are waiting for more piñatas to hatch or be fetched, send some off to parties if there is enough time to spare. Also, the twins sell for more than the regular piñata.
p.s., I saw someone get all of the hearts and beat the Whirlm maze with 12 seconds to go. They got the twin bonus PLUS they were wildcards.