Combining two or more models in ETABS
1) You can open two ETABS sessions say Model A and Model B.
Model A
Model B
2) Select entire model say in Model A, use Edit > Copy option or Cntrl-C
3) Now go to Model B and the use Edit > Paste Option or Ctrl-V to merge Model B into Model A.
4) Set coordinates and rotation for pasted model, you can see a preview of the model if you place the cursor over OK
5) Click Ok and the two models will be merged
6) Additional models can be added by repeating this process
Note: The procedure above assumes that section properties used in Model B are also present in Model A, and you may need to reassign loading.