You need the two values in separate fields. If you can't import it that way, you can write calculation fields that extract each value so that you can specify them for sorting instead of your combined field. You'll also need to specify Number as the return type for these calculations.
Left ( YourField ; Position ( YourField ; ":" ; 1 ; 1 ) ) will extract the first number
Right ( YourField ; Length ( YourField ) - Position ( YourField ; ":" ; 1 ; 1 ) ) will extract the second number