Other players vessel can not be viewed by other player. But refocus of vessel will make it viewable by other player
How to recreate on a single computer (windows 8 in this case)
KSP 0.24.0.549 (x64)
Dev version of DMP client and server tag: ed6a2f7
2 Computer Monitors
1:launch server
Launch KSP on one computer monitor windowed mode and another instance of KSP on the second monitor also in windowed mode
Connect to server with one instance of ksp running on one monitor
Connect to server (CHANGE user name) with second instance of ksp on other monitor
the way I did this was have a space plane on runway on one monitor and a rocket on the pad on other monitor. On my system both vehicles were viewable on one monitor but on the other they were not. I moved the plane up a bit on runway then I launched the rocket. Some abnormalities with the throttle controls happen and appear to be caused by the other client somehow. But after clicking with mouse on one windowed game and then back to other window again made both vessels appear and viewable on both clients

This is a stock bug, should be fixed in 0.24.2.