Two different Lord Vishnu Avatar were in Mahabharat Sequence. Who are they and how it is possible?

As per Amba's request, Sage Parashurama (Lord Vishnu Avatar) came to Bhishma and confort to marry Amba. As he delcined, the war between Parashurama and Bhishma went for 24 days and resulted draw. Later Parashurama went to Amba and informed about sequence. Lord Krishna is another form of Lord Vishnu who charitoeer the Arjuna. Here Amba came as Shikandi to kill Bhishma after plan plotted by Krishna and Yudistira.