For the sake of simplicity, let’s assume re-splitting aces is not allowed. Also recall that Spanish 21 uses 48-card Spanish decks with no 10’s. Finally, recall that 21 points is an automatic winner in Spanish 21.
Even without that rule, the player should always split aces. The probability of getting a pair of aces in a four Spanish-deck game is combin(16,2)/combin(192,2) = 120/18336 = 0.65%.
The probability each ace will get a 10 is (4*12)/(48*12-2) = 48/190 = 25.26%. With two aces, the expected number that will turn into blackjacks is 2*48/190 = 96/190 = 0.5053.
Each blackjack will be worth an extra half unit. Thus, the value of this rule is (120/18336)*(96/190)*(1/2) = 0.17%.
Even with this rule, the player should still not split tens, so we don’t need to worry about that. So, this rule lowers the house edge by 0.17%.