Dave,
The importend point in this discussion are the following:
If we have two shocks
- spectrum X at the lower floor
- spectrum X1 at the higher floor

spectrum X act on the support nodes (lower floor) 10,120,150 (dependent) spectrum X1 act ont he support nodes (higher floor) 115,125 (dependent)

With this nodes numbers the user must more as one cell for the spectrum X
write in the spectrum load case.
X 1 10,120,10
X 1 150,150,1
X1 1 115,125,10
This is then a independent shock ( because 2 cell input for X and not one input cell for X)

Its clear, we can solve this with the right node numbers (support/spectrum to attach to)create in the model, but this is very circumstantial.