how could the tapetal cells become bi-nucleate?

The innermost wall layer of microsporangium is known as tapetum. It provides nutrition to the developing microspores. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.

 Tapetal cells become bi-nucleate by multiple cell fusions of uninuclear cells (cells with a single nucleus). Thus, tapetal cells are syncitium ( cell having more than one nucleus).