Achievements of the guilds don't stack- they are compared and only the higher one is taken. E.g, guild A has room for 15 members, guild B has room for 25 members. When those guilds merge, they will have room for 25 members (since 35>15), but NOT for 35+15=50.
In your case- the maximum membership will be only 35 "seats", NOT 75 (nor 65).
