Simulating double columns - side by side welded all over the length?

One way to do this is to use member offsets. You will need to break up the columns in segments so that you can establish the connectivity at each of those intermediate nodes. You will have the nodes created first. These nodes will be common nodes for both columns. Create one column first by creating members connecting the nodes. Provide a positive offset ( = half of column depth). Create another set of members between the same nodes and then provide negative offset (=half of column depth) to those.

One way to do this is to use member offsets. You will need to break up the columns in segments so that you can establish the connectivity at each of those intermediate nodes. You will have the nodes created first. These nodes will be common nodes for both columns. Create one column first by creating members connecting the nodes. Provide a positive offset ( = half of column depth). Create another set of members between the same nodes and then provide negative offset (=half of column depth) to those.

Thanks for the reply, how small segments do you recommend ? the height of the columns is around 12m.

The intention is to make the columns deflect like a combined section and so any spacing that helps achieve that deflection pattern should work. I would start with say 2 m intervals, watch the deflected shape and bring it down to 1 m if needed.