It's a precedence issue: as written, the SameTest function is
TautologyQ[Function[Equivalent[Slot[1], Slot[2]]]]
which just gives False.
Try
In[10]:= Union[conjunct2, SameTest -> (TautologyQ[Equivalent[#1, #2]] &)]
Out[10]= {False, True, x, y, x && y, x && ! y, y && ! x, ! x && ! y, ! x, ! y, ! (x && y), ! (x \[Xor] y),
x || y, x || ! y, ! x || y, x \[Xor] y}