What to do with second Eir?

If you intend to use her in AR, then merge.
Probably the neutral into the +Atk -Spd one (only my opinion).
If not, fodder.
Swift Sparrow and her B-Skill are really nice fodder material.

Eir could do okay with -Spd, but I wouldn't recommend that path. With SS2 and Lyfjaberg's effect, this is really the difference between 42 ES and 45 ES, and it is a significant difference.

Being at 42 ES means that anything with more than 37 Spd is not getting doubled, and that is a significant portion of the roster (roughly 90 or so units fall above 37 Spd if taking Spd boons into account). If you have Neutral Spd, you get to 45 ES, which means that your threshold for doubling is at anything over 40 Spd, which is only about 15 units.

See the difference? Now there are 75 more units that you could potentially double, assuming no merges. The extra attack isn't so important when the ability to double comes into play, and it would be mitigated by the extra 8 attack on initiation. So you should absolutely merge into the neutral, if you do decide to merge.

You can do this :v

Depends on whether you do aether raids or not, if you care for score.
I have three myself(neutral,+att-hp, +res-def).
Still can’t decide on what to do with them.