Currently double tall flowers are impossible to get on skyblock.
A challenge that you achieve in the Novice, Competent, or Expert leagues can fix that.
Currently there are 10 different flowers in minecraft, not including the 4 double tall flowers. Collecting 2 of each flower should get you 1 of each double tall flower.

Thank you!

EDIT: So it's come to my knowledge that there's already a challenge similar to that on Advanced that you need to collect 8 of each obtainable flower. Right now you only get money and experience for the reward. This would be perfect to get rewarded the double tall flowers as well.