Flu strains are named after their types of hemagglutinin and neuraminidase surface proteins, so they will be called, for example, H3N2 for type-3 hemagglutinin and type-2 neuraminidase.
If two different strains of influenza infect the same cell simultaneously, their protein capsids and lipid envelopes are removed, exposing their RNA, which is then transcribed to mRNA.
The host cell then forms new viruses that combine antigens; for example, H3N2 and H5N1 can form H5N2 this way.
Because the human immune system has difficulty recognizing the new influenza strain, it may be highly dangerous.