Single and twin fin is a fascinating area. Going back to a number of projects, the choices have not always been obvious.
F-16 was two fins until quite late on, then jumped to one. F-14 was one fin and jumped to two. F-15 originally had (I am told) one fin and went to two. Early F-18 designs had one fin. And so it goes on.
The reason given by LM for designs like F-22 and F-35 was, in my view, as much to do with the aft fuselage structure and the difficulty of getting a single fin mounted above or in between the engines as much as the aerodynamic advantages. If you need two booms aft to mount horizontal tails, slapping the fin on the boom can look attractive to reduce weight. Twin fins are touted as superior to singles at high AOA, and especially where any yaw is encountered, where single fins can get severely stressed by vortices from leading edges (the Tornado F2 suffered from this). However, F-18 had the same problem with two fins, so go figure...
I have been told that the Tornado and Typhoon HUGE fins are down to the BAE chief designer at the time, who demanded high margins for directional stability at certain Mach numbers - I'm not an aerodynamicist, so can't be sure.
Two fins are a big help with signature - a single vertical fin will usually be a decent reflector, two canted fins can be engineered to reduce the RF return.
The Iranian aircraft is a straight F-5 copy, and any advantages gained by two fins are likely to be outweighed by the additional weight they will have to have incurred in adding structure on the aft fuselage to mount those two fins.
Regards
Engines