Should I disclose to the editor that I am reviewing a similar paper by different authors for a different journal?

Simultaneous or independent discoveries happen often. If the paper A has a first submission date published, that should ensure paper A being fairly treated.

Apart from that, I do agree with the other responses that actually discussing the situation with the editors (without disclosing mutually the authors) may be a good idea here.

My purely personal view (others may disagree) would be that independent results which overlap temporally due to an unlucky accident both deserve publication. It was pure coincidence that in your case they ended up going via a single-person bottleneck.

This exact situation is difficult to assess without detailed knowledge but the editors of the journals should be aware of the situation. Since each case is unique and we do not know the details it will be up to the editors to judge or take the matter further.

The Committee on Publication Ethics (COPE) has one case on simultaneous publications that can be of interst to read.

So the advice is: yes contact the editors and provide the information you can. They will then have to take action, compare manuscripts or whatever the deem necessary. Similar works have been published simultaneously before so it is not certain there is a violation lurking somewhere but it is unusual.

There are many cases of simultaneous discovery.

I don't think you need to interfere, unless there is reason to believe the second papers authors were referees on.the first, author overlap, dual submission or other plagiarism/unethical behavior. As far as I can tell, you have no reason to believe so?

See: there is a good chance the first (or both) will be in print before one could have read the other. Then both have studied this "first" (does not sound like major breakthrough anyway).