Non matching meshes

Hi,
Is there a way to handle non-matching meshes (with no overlaps) with freefem. I have a very basic laplacian test case, two squares meshed independly but sharing a common boundary (same tag). I read that mortar method could do the job using lagrangian multiplier to enforce continuity condition \int_{\Gamma}(u_1-u_2)=0, where u_i are the restriction of u on each side of the border but i can’t see how to do that… Any idea ?

best reragds.

PS : it’s a classical lapacian with u=0 on Gamma tag=1 and u=1 Gamma tag=2, dn(u)=0 on Gamma tag 3.
Processing: square.msh…
square.zip (17.1 KB)