Modelling of two chains

The X-ray structures of a protein has two chains (A, B) which forms after a proteolytic cleavage step of a single chain. i want to model proteins which have a single continuous sequence. However, I want to input the FASTA sequence as two separate chains A and B. How do I input the sequence?
Inputting a continuous single chain gives incorrect active-site due to conformational change after cleavage.