Is it reasonable to collaborate with a professor when you are employed by a different professor? How wise is it to collaborate with another professor while working as a staff under one professor? Is there a ethical conflict in doing so even if the other professor is from a slightly different area? 

Also, how should one develop his/her own expertise while working as a postdoc under one professor? Is it wise to also work with other professor with slightly different research interest?
In contrast to F'x's opinion, I think collaboration with people outside your group is not just wise, but actually necessary, at least if you are working toward an academic career.  You must develop both a wide professional network and broad independent research experience.  As long as everything is out in the open, with everyone's knowledge and agreement, there is absolutely no ethical conflict.

This does not mean that you should shirk your paid job; any outside projects must be on your own time and must not interfere with your official duties.  Similarly, you can't use your supervisor's resources without their explicit advance permission.  And your supervisor may be happier if there's some chance of involving them in your outside projects.  But ultimately, how you use your own time is your own business.  (Any supervisor who tells you otherwise, or insists that you don't have "your own time", is simply abusing you.  Run.)