Hi ivkicin,
I'm not too sure what programming environment you're using to communicate with these devices, but the download pages on the Keithley website have the software you'll need ( http://www.keithley.com/products/dcac/voltagesource/application/?path=6517B/Downloads#3 and http://www.keithley.com/products/dcac/sensitive/lowcurrent/?path=6487/Downloads#3 ). In regards to synchronizing these two devices, I would contact Keithley directly and see if they have any suggestions for you to start with on the hardware side. How much of a difference in measurement are you okay with?
Bobby Breyer
Applications Engineer
National Instruments