The 1x derailleurs use a very large offset between the cage pivot and the jockey wheel, so that as they wrap more chain on larger cogs, the rotation of the cage causes the jockey wheel to move farther from the cogs. This allows the derailleurs to "adapt" themselves well to different-sized cassettes, with the jockey wheel essentially choosing how low it sits based on how big the current cog is.
The problem with this scheme in a 2x arrangement is that changing between the chainrings also affects the rotation of the cage, and thus effects where the jockey wheel sits relative to the rear cogs. This can affect rear shift quality.

2x derailleurs solve this by relying more heavily on the parallelogram to manage where the jockey wheel is relative to the cogs, using little to no offset between the jockey wheel and the cage pivot.

If a 2x derailleur can shift a cassette just fine in a 2x scheme, it will also shift that cassette just fine when used in a 1x scheme. The issue will be if you want to use a cassette that the 2x derailleur isn't able to handle very well.