  We consider the fast monitoring of voltage collapse margin using
synchrophasor measurements at both ends of transmission lines that transfer
power from two generators to two loads. This shows a way to extend the
monitoring of a radial transmission line to multiple transmission lines. The
synchrophasor voltages are combined into a single complex voltage difference
across an area containing the transmission lines that can be monitored in the
same way as a single transmission line. We identify ideal conditions under
which this reduction to the single line case perfectly preserves the margin to
voltage collapse, and give an example that shows that the error under practical
non-ideal conditions is reasonably small.
