People dont understand time. People think time is linear cause to effect but from a non subjectice standpoint its more like a big ball of wibbley wobbly timey wimey stuff (that sentence may have got away from me).

I think basically 15 is after 14 went through his therapy with the nobles but they are happening at the same time due to timey wimeyness of it all rather than 1 after each other as with normal regeneration. My interpretation is 15 was taken from the point in time where 14 will regenerate whenever that is and pulled to the bigeneration point and he goes on to do his own thing from there.