Yeah, you can. Basically, every time you attach a toy to a spline, you can specify the offset from the starting point, in each of the three axis. So, you can set it up so you put a Square Block at the start of the spline, and a second Square Block four units above the starting point, so in practice both of them are stacked.
It takes a bit of practice to determine the right offset, but it's completely doable
The opinions posted here are my own, and not my employer's.