I just installed the 64 bit version of 8.5 (been using the 32 bit version until now). For some reason, the synth property page of each plugin opens in a new window outside Sonar (as though it's a separate program, instead of within Sonar as with the 32 bit version). Is there a way to confine them all to the Sonar window?

If they are 32bit synths, they are running in a separate program, the BitBridge.

OK, that's what it is. Does the 64 bit version of X1 use the same program as 8.5 for 32 bit plugins?

Every 64bit version of SONAR uses BitBridge. BitBridge was last updated in Nov 2009. There is a 3rd party alternative, JBridge, that some use instead of BitBridge.