They are two separate accounts, at least they were when they were First Active loans, not sure what UB have done with them. First Active account numbers have been changed to fit the UB system so somewhere the dd you are paying is being redirected to the two original loans that you have.
It's not that it was one account number, the main number for all a customer's accounts was the same, it was called a customer number rather than an account number, the end bit then denoted what type of account and which one it was, for example you could have 4 mortgage accounts for 4 different properties or on the same property, they would all start with the same customer number but would have a different ending, for example 9401 would mean mortgage no.1, 9402 no 2, 9201 top up no. 1, 0101 investment account no1 and so on.
I doubt you have an option to make all the repayment go to the higher one as both would have repayment terms and amounts on them. They are both separate mortgage accounts secured on the same property. If you just stopped paying one in favour of the other then you would be in arrears on the unpaid one. These are totally separate accounts, not sub accounts. Your mortgage was structured in that way as a split fixed/variable for whatever reason at the time. Not uncommon to go for the option of half fixed half variable, kind of spreads your bets, rates rise you win on the fixed, rates go down you win on the variable.