Working in two research lab at the same time – unethical?
So my question is, is it unehtical for me to do this?


Unethical - no. Smart? Probably also no.

Firstly, as Corvus correctly states, it seems very unlikely that you can perform well in both roles and do your undergrad studies in parallel (and I am not even talking about going outside and doing people stuff occasionally, which you should also not forget about).

Secondly, you should not forget that professors are human. Even though you are not doing anything "wrong" by any stretch of imagination, it is not difficult to imagine scenarios where either of the professors (or both!) don't take it well, and I am assuming you want letters from these guys down the road. The professor that you work for, for instance, may assume that you find his research very unfulfilling if you feel you need to invest your spare free time into a completely unrelated area instead of hanging out with friends etc. (S)he may assume you are really doing it only for the money / letter. The other professor may be disappointed at some point when an important deadline should be met that you can't commit more time to her/his project, because of your other duties.

Finally: 


  Professor A doesn’t know about this.


That's never great, and you should change this ASAP. Even if the professor basically does not care about the issue at hand, (s)he may get mad simply because you didn't tell (I certainly would). And I think you are aware that the chance of you keeping this a secret forever are slim to non-existent?