The court must decide the apportionment of money between the judgment debtor and the other person named on the account. To successfully obtain a garnishee order, the court must be satisfied that all the money in the account belongs to the judgment debtor. This can be established by looking at the origin of payments into the account.
It will only be possible to obtain a garnishee order against the bank if the court is satisfied that none of the money in the account belongs to the other named account holder; effectively, they are dormant.