Can I place 2 or more bids for each seller? That is, if I can not buy the first offer I have a second offer.

As I already responded in the support ticket, you have to use Groups for that:
https://www.myibidder.com/login/faq/2/#q9

If there is something not clear or you need more information, please follow up instead of asking the same question multiple times, otherwise it may add extra confusion for you.