Troublesome with surfacing with these curves

Good evening / day to you all!
I’m new to Rhino and I have an assignment that consists in reproducing the Tverrfjellhytta pavilion in Norway.

I’m using only Rhino (so no GH) and I had been suggested to use the Tsplines plugin.
For those who are not familiar with the pavilion :

So far, everything is going pretty well except when it comes to the upper part when the two separated “wood sections” merge into one unique piece above the door.
What I have been doing is simply draw a few curves at the propre height for the general levels (the seats inside the pavilion) and then create surfaces with them so I can convert everything into Tsplines. Then I shall be able to play with the control points.

I have been using Loft to creat surfaces between the lines, wich is fine, but how can I converge those two separate volumes into one like we see in this photo?

It seems like Loft cannot take two curves that are on the same level so they can merge into a unique curve that is higher. I succeded in making two, but I don’t want this ‘crack’ dividing those two. I want it to be one unique organic form.

To me it seems like it would be a job for Patch, but it’s not working either … it’s pretty much just making a gigantic mess.

What can I do now …?

Thank you very much for your help in advance!

A rhino apprentice