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.