Q: synching monitor

Are you asking if you can use your 2008 iMac display as an external display for your 2015 21.5" iMac? If so, then unfortunately you cannot unless you use VPN however the performance will be awful. Your 2008 is simply too old.  What you are describing is called Target Display Mode, Apple explains what iMacs TDM works with in Use your iMac as a display with Target Display Mode - Apple Support.

Your best bet is to buy any external display and use the 2008 iMac for something else.