Forms doesn't support running the application pool with more than 1 worker processes as Forms store some session data (form cache, access rights cache, submission cache ect.) in the memory and these data can't be shared among multiple processes even after you change the web site session state mode from "in progress" to State Server or SQL server as they are Forms application level session not IIS ASP. Net session. 

Also per https://blogs.iis.net/chrisad/1342059, the Web Garden is designed for "Offering applications that are not CPU-bound but execute long running requests the ability to scale and not use up all threads available in the worker process.” while Forms is not such application so it is not suitable to use Web Garden for Forms.