Ah sorry i missed the grasshopper part.
If you absolutely have to join them in grasshopper and can’t use the native Rhino _Join command before referencing, there is no native way to do this, as the problem is in how grasshopper defines the tolerance for join operations.
In the attached file is a small GhPython Component, which joins the surfaces using double the file tolerance, which is aligned with how the native Rhino _Join Command does it.
Hope to help
joinBrepsTolerance.gh (8.1 KB)