Hello Chris,
To be honest I do not know if that's posible since every license has its own PAK (Product Activation Key) and activation key associated with the ASA serial number. Meaning, that they would need to create the same PAK and activation key for both licenses in order to merge them into one activation key. I think that's why Cisco has already established the amount of licenses for every package. Also, I do not know if will be cheaper to buy two 10 package or one 25 package. Those are things to consider, as well some extra licenses in case you need to add more users.
Cisco AnyConnect Premium or Essentials:
(P/N: (L-ASA-SSL-***=) 10, 25, 50, 100, 250, 500, 1000, 2500, 5000, 10,000
It would not hurt to ask any Cisco Sales Engineer if there is any possibility. But I really doubt it.
I hope this helps,
Luis.