  In this work, we discuss the improvement that the joint network of LISA and
Taiji could provide on exploring two kinds of black hole formation mechanisms.
For astrophysical origin, we consider light seed and heavy seed scenarios, and
generate populations accordingly. We find that the joint network has the
potential to observe growing light seeds in the range 15 < z < 20, while single
detector can hardly see, which would shed light on the light seeding mechanism.
For the heavy seeds, the joint network only improves the signal-to-noise ratio.
For primordial origin, we calculate the event rate at z > 20 and detection
rates of LISA and the joint network. The joint network expands LISA's horizon
towards lower mass end, where the event rate is high, so we have better chance
observing primordial black holes with the joint network. We also estimate the
parameters using Fisher matrices of LISA and the joint network, and find that
the joint network significantly improves the estimation.
