first of all do a structure like this
then, do 2 control sectors,like this,i like to use triangles, if you want you can use squares or any other shape
now, right click a linedef of the triangle and put this action:
linedef type 432 (it has to be tagged 0)
yay!,now we are finished with that linedef, lets pass to the next
then in the other one put action 300 or 301 and tag it with the number you want(if you want a new one just press new in the identification part).
ok that's all with the linedefs to enter 2d,now mark the wanted sector with the tag you gave your linedef executor.
well the enter sector is done but don't forget to mark it with special 64
now do the same with the exit sector (obviously with a different tag)
in this part now put exit 2d to make it exit 2d mode,welp,thats all i can help,hope it works