Is it possible, and in case, how, to switch focus from one display (with "display" as in $DISPLAY), to another (within the same X server) without using a mouse?

Example: An X-server running two displays on two different monitors. How can I change focus/move cursor from one monitor to the other?

I use typically Fvwm2 or Sawfish on XFree86 version 4.2 on Linux-2.4.19.