Suppose I am creating a large image files out of smaller ones. The original images (jpeg) already have chroma-subsampling of 4:2:0. If one were to put them together and save again as a jpeg, would saving it with chroma-subsampling of 4:2:0 (as opposed to 4:4:4) have a "double" effect, similar to re-compressing an image?
When not using chroma subsampling, JPEG generational losses are pretty much limited to rounding error as long as the same compression settings are used. (Majority of compression losses occur during the first compression.)
Here is what happens when an image is recompressed until convergence at q=90 without chroma subsampling (original, 1, n, difference):
However, when chroma subsampling is enabled, additional color information is thrown away, which probably also affects quantization, so that losses are much more visible when steady state is reached. Here is what happens with chroma subsampling enabled.
If you're concerned about single-digit resaves, the effect isn't so pronounced.