What regulates these two methods of splicing?
As Alan Boyd said, this process is called trans splicing where exons of different RNAs are joined together. Physical proximity is conducive for trans-splicing. This study shows that trans-splicing frequency is higher if the genes are proximal. Proximity not only means closer in the genome but also the 3D proximity governed by chromosomal interactions. 

Now, what factors  promote trans splicing is not properly understood.