Can I be work on several project as a post-doc?
It's quite common for postdocs to work on different projects at the same time, but this has to be agreed with the PI who is funding you. 

In my experience, there is a general understanding in academia that contracts are not strictly limited to the project being funded. For example, postdocs are often expected to apply for funding and this is rarely officially included in the scope of the project. Another very common case is publications: the potentially long process of publishing a paper sometimes overlaps with the next contract, but in general PIs are fine with letting you finish papers related to your previous contract.