"Edge of Eternity" is one of the first games that have FSR and DLSS I assume.
This is an interesting interview with the game developer about the difference between both.
It looks indeed that the difference between FSR and DLSS may not be worth it to implement both.
wccftech.com
Cost to add it
I wonder if this will have time involved because that introduces artifacts, so may be harder to get right, certainly if you compare it to a good FSR 1.0
The likely comment for FSR2.0 will then probably be: looks a bit better than FSR1.0 but has sometimes extra artifacts.
This is an interesting interview with the game developer about the difference between both.
It looks indeed that the difference between FSR and DLSS may not be worth it to implement both.

Edge of Eternity Tech Q&A - Midgar CEO on AMD FSR vs NVIDIA DLSS, PS5 vs XSX and More
We interviewed Midgar CEO Jérémy Zeler-Maury, also the Lead Programmer on Edge of Eternity, one of the first games to support FSR and DLSS.
Cost to add it
QualityI think FSR being very easy to integrate means more games are going to get it, DLSS is more complicated to integrate.
I didn't expect him to say this below, since now people assume DLSS2.0 is still always better:Quality-wise, when there is a lot of pixel info available for upscaling to 4K, both technologies give amazing results, and I have a hard time seeing differences between them. I’d even say that I slightly prefer the FSR for 4K resolution since it doesn’t introduce any artifacts/minor blurriness that DLSS can sometimes introduce. For lower resolutions like upscaling to 1080p or 720p, I think DLSS gives a better result since it can reconstruct parts of missing details due to the nature of the technique.
From another interview with AMD it was clear they were already busy with FSR2.0I slightly prefer the FSR for 4K resolution since it doesn’t introduce any artifacts/minor blurriness that DLSS can sometimes introduce
I wonder if this will have time involved because that introduces artifacts, so may be harder to get right, certainly if you compare it to a good FSR 1.0
The likely comment for FSR2.0 will then probably be: looks a bit better than FSR1.0 but has sometimes extra artifacts.