The endosymbiosis theory is supported by all but one piece of modern-day evidence, which is both organelles

Assuming that you are looking for supporting evidence, so here are two of them:

1. Both mitochondria and chloroplasts (said to have become organelles through endosymbiosis) have their own DNA different from the cell's nucleus.

2. During a cell's division, mitochondria and chloroplasts divide independently by binary fission.