Hello @lucas12,
Which PS version are you using? Because in my 1.6.1.24 version the new cart is kept.
EDIT: Now I understand. You want to combine the last cart he abandoned, with the one he just created. Except of the fact that this does not happen in any e-shop (if the client needed his cart he could simply login) it is also consists of a really poor user experience. Think of this user case:
1. User creates a cart with 3 x A items.
2. User abandons cart and logouts (for whatever reason).
3. User comes back, forgets to login creates a cart with 5 X A items.
4. User logins.
How many A items should there be in the cart? 3? 5? 8?
I suggest you stick with how Prestashop (and most eshops in the world) works. You should NEVER a normality for a specific user case. If what you said ever happens, you can always help THAT specific user out by locating his old cart manually and creating a manual order.