Two Questions

Still keep them, if you really manage to get both tamamo lancer and squirtoria from the summer banner (which you should need at least about 600-750sq for on average, just so you now what you should prepare for), 1 second archive of exp cards is not gonna get you very far considering a 5* servant needs 375 class specific EXP cards to get from 1 to 90 (so 750 for 2 5* to get from 1-90, but only if you only use class specific EXP cards, for non-class specific one you would need 450 from 1-90)....and even a fully expanded second archive can only hold 200 cards, so nowhere near enough, even if you should only manage to get one of the 2....and that isn't even taking summer shishou into the equation (wellfare). So you see, you really can't have enough EXP cards for the upcoming banner and they're much to precious to waste...