  We measured the magnetoresistance of bilayer quantum Hall (QH) effects at the
fractional filling factor $\nu =2/3$ by changing the total electron density and
the density difference between two layers. Three different QH states were
separated by two types of phase transition: One is the spin transition and the
other is the pseudospin transition. In addition, two different hystereses were
detected, one of which is specific to bilayer systems. The phase transitions
and the hystereses are described well by a composite fermion model extended to
a bilayer system.
