Lets say there is a young man who has been in the seminary for his (arch)diocese for year, and is deadset on ordination. Would it possible for him to do a semester or two of his studies at another house of formation? Example, would be permissible for him to continue his studies, but for a year do them in an FSSP house of formation, or Jesuit or Dominican community?
I imagine that this would depend upon the (Arch)Bishop, the director of vocations, and the dean of the seminary, as to whether this would be allowed.
The format and method of formation can be very different between religious orders and diocesan seminaries. Granted some seminarians study alongside religious novices but intellectual formation is of course only one aspect of seminary formation. Any order would have to consider whether accepting someone only for a year would have a negative affect on their community - which is of course the very thing which makes religious orders distinct. as someone has already said, this would be very much up to the Archbishop, seminary formation staff and the order itself. I can see (in theory at least) a short period of "alternative discernment" being allowed although what effect this would have on the seminarian's studies is another question.