I have occasionally heard of people getting more than one ticket applied to an upgrade. The only instances I have heard of that worked were when all of the following applied: only one of the tickets being used for the upgrade had been used, the "used" ticket was still valid, and the total value of the tickets did not exceed the cost of the pass being upgraded to.
I really don't understand why you want to do this though as I see no advantage. You will only get credit for what you actually paid for the tickets toward your upgrade, so even if you get a discount it won't help. Also, if you use the first 5-day ticket completely then start to use the 2nd 5-day ticket and then try to upgrade, you will still only be able to apply credit from the 2nd ticket since once a ticket is used up it no longer holds any value. If you want to use both tickets for entry on the same day to rack up fastpasses, a scheme an acquaintance of mine once asked me if I thought would work, again you won't be able to apply both passes to the same AP upgrade since both tickets will have been partially used and from Disney's perspective would be doubted as belonging to the same person.