A risograph will not put the second colour exactly where the first one went. The drum shifts, the paper stretches with humidity, and across a run of forty sheets you get forty slightly different overlaps. Offset printers spend real money eliminating that. We spend the run leaning into it.
Designing for the error
The practical consequence is that you cannot draw a two-colour image that depends on a hairline meeting another hairline. What you can do is design shapes whose overlap region is itself interesting — so that a two-millimetre drift reads as a deliberate third colour rather than a mistake.