Like most things in Max there are several ways to do a lot of the same tasks. Here's a kind of simple and manual one that will get the job done. Someone else may see this and say, "hey, there's a much better way to do that" and I welcome those as it gets some dialog going and we learn new ways people work.
Watch the video I made. It's essentially this. Make sure the normals on both planes face out. Then, grab the edge of one plane and slide it to intesect the other. Then grab the other plane's edge and pull it up to the intersection. Attach both planes together and weld the verts. Done.
It's a little imprecise, but many people who model for architecture do things like this by hand because the world is not perfect and in the real world there is imprecision. So if you intentionally inject a little imprecision into your models, they tend to look more realistic.
I hope this helps.
Rob Holmes
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3ds Max (2023-2025), V-Ray 6.2, Ryzen 9 3950-X Processor, DDR 4 128MB, Gigabyte Aorus X570 Master motherboard, Sabrent Rocket NVMe 4.0 M.2 drives, NVidia RTX 4090, Space Pilot Pro, Windows 11 Pro x64, Tri-Monitor, Cintiq 13HD, Windows 11 x64
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