Next TamaCas & Cu Alter Solo Rate up summon Banner

Tamamo's next solo rate-up is during the Fate/Extra CCC pre-campaign banner which rotates with Gil, Karna and Nero Bride's rate-ups.
Cu alter doesn't look to be getting a solo rate-up this year at all.
Source: https://grandorder.wiki/Upcoming_Summoning_Campaigns_(for_EN_reference)

From what we know from JP, Cu Alter will not be getting one, indefinitely. Your best shot at him is during his Chaldea boys rate-up, which he shares with Proto Arthur.