Copy Paste Sectors in Blood’s mapedit. This won’t just be making two sectors, this will be pasting a sector inside of another.
Hold Alt and hover over the desired sector.
Press Insert to make a copy of the Sector
Drag the Sector to your desired location and press ALT to place it.
Draw Sector around your pasted Sector.
Connect the vertices from the new sector to the pasted sector
Press J to join sectors and get rid of the messy sectors you don’t need. Press J outside the sector and J inside the sector you want to connect it with.
Watch the video also: