Linear elastic material

Hi,
I implied two linear solid mechanic (linear elastic material) in two rectangular(( Rectangular1)and rectangular2 ) ), there is a common inner boundary( number 2) between these two rectangular, i imposed the displacement of 1cm at this boundary 2 in the fallowing figure to see the behavior of the mechanical stress:
My problem is that when I use the identity pair for the boundary 2 and then use continuity for it , I got this error :
"Illegal domain selection".
- Pair: p1
I---------------------------------I(1)
I--------------------------------I (2)
I---------------------------------I(3)

thank you so much