Based on the question about How are "A Certain Magical Index" and "A Certain Scientific Railgun" connected?, these two anime are the same. Or so I understand the answer, maybe I got it wrong?
And I saw a question about Should multiple continuities have individual tags? and according to Kuwaly's answer,
For Neon Genesis Evangelion, and in general, I think having a policy of fewer-is-better should be used when regarding tags
Don't you think the same policy should apply for Railgun and Index? Can't we just make a synonym or something?