If they are in different layers you can adjust the layer intersection number to be different for each layer and they will not trim.
You need to set this for each layer combination where it is important for them not to trim.
If they are in the same layer then you can set the layer intersection number to zero (0) and they will not trim - but neither will other walls in that layer.
Or move the reference line of the thick wall to the other side (Design > Modify Wall > Reference Line...)
Pull the end of the thin wall away from the thick wall.
While the thin wall is selected and the wall tool is active, CTRL-click on the non-reference side of the thick wall.
The walls will touch but not trim.
If you want the walls to overlap then draw a line where you want the thin wall to stop and CTRL-click to that as above and delete the line afterwards.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11