Strangely it will only project a single curve from the entwined curve onto surface 1. Any help would be appreciated and I can upload my script if needed.
After some trial and error I finally worked it out… the script itself was correct but it had to do with the ‘absolute tolerance’ within Rhino preferences set too fine at 0.01 millimetres. I bumped it up to 1 millimetre and reran the script in Grasshopper and the brep join component successfully joined both surfaces together