I can't say exactly what causes your geometry to fail, so let me give you a simple example that accomplishes what you're trying to do.
Try this sample 2D script and see if this approach would work for you:
hotspot2 0, 0, 1001, half, 1
hotspot2 half, 0, 1002, half, 2
hotspot2 -1, 0, 1003, half, 3
A=half*2 ![edit] you don't really need this line, see below
line2 -A/2, 0, A/2, 0
hotspot2 -A/2, 0
Now, this sample script defines length of line by its half length. To get the overall length updated in the dialog box you may use the following Parameter script:
parameters A=half*2
lock "A"
Now, say you want to alternate between being able to define the length BOTH by "A" and "half", I suggest you program a switch, make your calculations conditional (IF... THEN), and let the user choose. ArchiCAD will not let you alternate between them in a continuous loop.
