Correct me if I'm wrong, but so far the only way I've been able to have a two day event, is by creating a new, separate, event for the following day and just naming it as such.
The issue this causes is two-fold: superstars complain about not being on whichever of the nights they aren't booked on, and (in 2016 at least) the second event got penalised as it was seen in the game as holding events too close to each other.
To get round the first issue, you could make the event branded, with each brand being shown on each night which is what I plan to do for my Supreme Challenge, but that isn't ideal, and I still expect to get dinged for the event being so close to the last event, in the game's eyes.
My suggestion would be to have an option within an event to make it a two day affair, much like Wrestlemania was this year. I believe this would be much more realistic, and mean that you don't have to do so much of a workaround to get the result you want. It would also mean the game seeing the two nights as part of the same event, so avoid the ding that was the case in 2016.
There is probably a reason this hasn't been done, or maybe it's possible just not through the event screen (but I've scoured to find a way), but I just thought I'd suggest this here.