In 1st AC (and 1st class), the berths / seats are allotted only on charting.
Being the highest class, it is VIP prone and they are given the maximum comfort.
After the VIP absorption, it comes to the general passengers and if available, a coupe of 2 may be given to a PNR of 2.
Yet, suppose, on a day in a...
more... train, after VIO allotment, remaining there are a coupe of 2 and a coupe of 4 for 6 un-allotted (so far) passengers who have 3 PNRs of 2 each. The charting clerk can now give a coupe of 2 to only one PNR and rest 2 PNRs (of 2 each) will have to be accommodated in the coupe of 4.
This way, there may be a chance that a PNR of 4 passengers is bifurcated into 2 or more Coupes.
Ladies traveling alone are also given preference to be either in a coupe of 2 with other single lady or to be there in a coupe of 4.
Wish you all the best for allotment of a coupe for 2.