Okay, so living with you for 50% of the time would be a massive change to their daily routine. In my view it is doubtful that the court would consider a 50/50 shared arrangement in these circumstances, as it will be a bind for the kids to have that level of travel on weekdays.
That said, it never ceases to amaze me as to what some judges/magistrates will order, so if you and the mother cannot come to an agreement between you, and mediation also fails to help you come to an agreement, your final option is to apply to court and see what a judge/mags have to say about it.
Bear in mind that the court is supposed to have the children's best interests as paramount (which is why I say on the info you have given, a 50/50 shared care order is unlikely), so your application may fail on that basis. Also, at 14 years of age, your older child's views will be determinative to the courts - the younger child's views will be of interest, but not determinative.
If you and the mother have managed thus far amicably without the use of the courts, I would strongly suggest that you continue to try to do that, as court has a tendency to make people say things that they would not normally have raised. Mediators are a great help, and much cheaper than going to court with lawyers.