Merging to Programs into one ? i keep getting Assembly ref errors

Question

How would one go about merge two programs (two different forms ) into one project ? It seems its not as simple as just Adding the items needed... and then taking out the static void from one of the .cs files and running the form for the other cs file... Anyone have any sort of strict guidlines as to how to merge projects ? (okay i only started programming in Dot.Net like a month ago so yeah noob of note)