Ah yeah, that is the case. The event images from the app should be already updated based on the new, corrected date range. Sorry about the confusion.
That's what the Frontier Coin purchases are for!
JJJ428 also pointed this out, but to be clear, the Lord of Light will have extended availability during this event to ensure that all 3 days of the event have the Lord of Light dungeon open to collect event fragments.
Technically you can use two of the same Gemini on one team (eg Gemini Pollux and Gemini Pollux) and get the extra benefits of the Dioscuri passive/related skills, but we still wanted to give people a guaranteed road to get both characters for this event, just in case.
We view these Lord of Light fragment events as one of our more accessible opportunities for players to get new, exclusive characters, so taking away the chance factor for the souls (like the previous Lord of Light fragment events) was something we wanted to still do, even if it is just a 50:50 coin flip where many people already earn half a dozen or more of these souls.