Just confirmed that you can get more than one character slot expansion item on your account but its limited to 1 per character that you have. So your 2 starting characters can make 1 each letting you have 4 characters total then your 2 new characters can also make 1 each bringing your total to 6 character slots. Just repeat the cycle again until the event ends if you're greedy for more character slots.
Also for those who have an extra raven/eve card, you can just use the card so that it wont take up any space in your inventory. I used a Raven card way back last March even if I don't have anymore character slot hoping that if ever they give a character slot event, I would be able to make Raven. Well, it paid off... Now I have Eve and Raven on my 3rd and 4th slot.
In the next 3 days I'll be aiming to get my 5th and 6th character slot while leveling my newly acquired characters.