If you mean two boards next to each other as in AB, you delete B and save A as one name, open AB delete A and save B as another name, then open A extend new board C and merge B so you have ACB.
Or do you mean something like converting seven baseboards making a C shape to eight baseboards making an O shape?
If so, then just use the normal extend function to create an additional baseboard and click in the gap. You might need to adjust the camera position and angle so Surveyor senses what you're trying to do. I just tried it in TRS2004 and it works fine; you can also fill in the middle to make a complete 3x3. I'd be surprised if it wasn't identical in TRS2006 and TC.
Sorry if I've misunderstood what you're trying to achieve.