If you own both utilities and your opponent throws 1/2/3 doubles do they pay 10 times the total on all the throws or just the last throw.
Just the last throw, as shown on the deed card:
If one "Utility" is owned rent is 4 times amount shown on dice. If both "Utilities" are owned rent is 10 times amount shown on dice.
"amount shown on dice", not "amount of all rolls this turn."
This is further reinforced on the Wiki:
If ONE Utility is owned, rent is 4x the number on the dice which landed the player on the utility, but if BOTH Utilities are owned, rent is 10x the amount shown on the dice.
(and with the analysis that the rent paid if both are owned averages to $70 , also given on that wiki page).