What you have to do for this achievement is simple just long term.
1.Become a master romancer for Swanana
2.Become a master romancer for Rashberry
Once you are a master romancer for both species you will unlock the mystery home from willy builder's shop. Put a mystery home in your garden. Now you need at least one swanana and rashberry, give both of them romance candy(if they are not ready to romance) and then send one to the other. Now they will romance once they have their baby you will unlock this achievement.