Duo Marth & Caeda will probably get the same treatment as Duo Ephraim & Lyon, that being in the void and limited to especific banners.
I do not expect them to be in the Monthly Schedule because that wouldn't make sense.
If I would guess, maybe Marth returns in August for the half anniversary Hero Fest, or next year for the Anniversary Hero Fest. Then maybe they could join the New Heroes Return schedule, which makes more sense than being in the LME rotation.
Since I didn't expect this to happen, I am 100% not ready for this banner. I may try to get at least the spark with the remaining orbs of this month plus the orbs from the first half of May, and try to get as many copies my luck allows.