When attempting to change the option for Two-Sided Material for a Material Instance, the option is initially greyed out and you cannot change the value. However, if you close out of the Material Instance and then re-open it, the value will be changeable.
Tested in //UE4/Release-4.26 @ CL 15973114
The Two-Sided option is greyed out and the value cannot be changed
The Two-Sided Material option isn't greyed out and the value can be changed
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-117794 in the post.