If the testing liquid reacts with the substances separately, it will react with them together as well.
For example, if your sample contains Ecstasy (MDMA) mixed with Speed (amphetamine), both will react simultaneously resulting in a muddy color with the Marquis Test. With Ecstasy alone, you’d expect a purple/black color.
Reaction of one drug can be much darker and more intense than another. If this is the case, it can be difficult or impossible to see the reaction of the second drug. This is a limitation of colorimetric testing.