You could try to merge nodes at the interface. This works fine if the meshes are similar. Just enter a global search distance, 0.1 mm or something.
Another way would be joining the parts via partition. This produces two volumes sharing surfaces (I think umit suggested similar procedure)...
The constraints created in modeler or assembly task won't affect the FE model (as LeonEng mentioned).
As you now have a FE model with two blocks with initial gap, use for example the springs to connect them. Also restrain the other block. For both pick only nodes to make sure that desired...
To be more specific:
You have an assembly with two blocks. Create FEM's for both parts and mesh them. Put the assy to the workbench (no FEM's). Then in meshing or boundary conditions task choose 'Create FEM from assembly'. Specify a name for a part to be created (I-DEAS creates a part with...
You'll have to constrain the two parts together after creating the FEM.
As a guideline, you can create FEM's for both parts with restrain and load set etc. Then after creating the assy FEM combine the previously created sets and join the two parts together with beams, contact, constrains etc.