Unless you use a BSM's Ingenuity, it will take you 3 Careful Synthesis II's to get a Twinthread finished. With Ingenuity, it drops down to 2. If you're daring enough, you can try using Steady Hands II + Rapid Synthesis (80% chance of success with SH2) / Standard Synthesis (100% chance of success with SH2) to oneshot it

But yeah, it's hard to HQ Twinthread (or any 1 star 40/40 craft for that matter) unless you have a good mix of cross-class skills, HQ materials, and good equipment/materia builds