Saber Dance will overwrite Fan Dance, and Fan Dance will overwrite Saber Dance.
Saber Dance boosts double attack rate to 50% immediately after usage, and it decays to 20% over the next 30 seconds.
Average Double Attack rate with 1/5 merits is 21.5% (assuming 5 minutes per usage) while average for 5/5 merits is 22.5% (assuming 3 minutes per usage).