No, unfortunately FPC Plats don´t get access to the Swissotel Exec. lounges. The benefits you get at the other hotel are quite limited: local calls, newspaper, welcome amenity, health club access etc. However, I´ve heard that FPC Plats occasionally get comp. upgrades to an Exec./Club room, but no idea whether this happens often.

While Swissotel and Fairmont are owned by one company, they´ve maintained two destinct brands for a good reason:

Swissotels are mostly 3.5*-4* hotels targetting mainly business travellers, Swissotels generally provide decent to very good service, but rarely are "the" hotel at the destination, or even among the top 5 hotels (the only somewhat famous Swissotel is the Stamford in SIN, and maybe the Howard in London)

Fairmont also targets business travellers, but has a much stronger focus on the affluent leisure traveller. Fairmont hotels generally are in the 4*-5* category, not all Fairmonts are the "top hotel in the area", but many are.

Therefore the focus of the two loyalty program differs: Swissotel focuses on free nights for use in the future (likely earned by stayed on the company´s dime) while Fairmont focuses on making your current stay better (room and suite upgrade certs).

Basically, if you intend to stay at Swissotels on a regular basis, I´m sure their program will be better for you. You could always try to maintain FPC Premier (which provides some decent benefits) for the odd stay at a Fairmont.