Not to jump on the bandwagon but I have to agree with wardogs response. I also don’t advise mixing species or even differently sized individuals of the same species in the same enclosure since that situation creates the very issue you are looking to resolve.

However, I will answe the question you asked in two parts.
1. The least invasive method is to feed at night in the enclosure when the red eye is active. I would place feeders in a 16 oz deli cup or similar container and watch to see if it feeds. This may take multiple tries and lead to some late nights.
2. You could also transfer the red eye to another enclosure just for feeding purposes preferably at night and monitor it. This is a method I’d least t recommend since it will likely stress the frog and may inhibit it’s natural feeding response.

Again the preferred method is to keep both species seperate and you’ll never have this problem, but if you are going to keep them communally you could try the above methods.