Dual-Cursed Oracle allows you to pick two curses, but the second curse only gets its basic effects and none of the bonuses when you reach levels 5/10/15. The RAW of Dual-Cursed Oracle says that the second curse "never changes its abilities as the oracle gains levels."
The Wrecking Mysticism curse lets you take extra Mystic Tails instead of mystery spells as you level, but the ability to do this is built into the basic effect of the curse.
If you take Wrecking Mysticism as your second curse as a Dual-Cursed Oracle, do you still get the option to take more tails instead of mystery spells or does the RAW of Dual-Cursed Oracle shut off that ability since it's technically an improvement from leveling?