I'm helping a friend and client set up his new SPLAT based studio.  Most things are working as normal but yesterday when showing him how to record an example project, no matter what I tried to solve the problem, trying to record any other midi track in the project would also trigger the EZDrummer2 track as well.  Even when it was not selected, soloed or even using the same midi port.  And while muting it, of course, would work...then he had no click track to record against.  
 
The odd thing about this is that it happens to me in my broadcast studio as well.  Randomly and infrequently.  Usually if I just shut down and restart it goes away.  But yesterday even doing a reboot of the DAW didn't stop it.  
 
Any suggestions??
 

Hi. Sorry if obvious, but is "Echo Input" turned off on the EZD track?
All the best.

Also check the channel numbers to make sure the MIDI track isn't feeding both synths (if Zargg's answer doesn't fix it).

Whenever this happens to me the first thing I check is if the track that's receiving the MIDI is set to Omni. I think there are still MIDI crosstalk problems in Sonar related to plugins which have MIDI-out capability, and if anything's set to Omni then it will pick up this data. I've seen some very weird stuff involving, say, having a bass synth pick up the MIDI from a drum track, and it's very annoying and hard to pin down.

I used to use simple instrument tracks exclusively and mostly still do, I also used to uncheck "ask this every time" when inserting a synth to be quicker, but for the problem in OP, what's happening is some synths like kontakt, dimension pro, default to enable midi out, which in turn causes this problem, so I now leave the box checked to say ask this every time, and right then and there when I insert a synth, I can see if in fact a synth is enabling midi out. of coarse just uncheck it if you don't need it for special occasion chaining midi to arps or chord players etc, I usually never do so I always uncheck enable midi out.  you can do this after the fact as well by right clicking on the offending synth in the synth rack view and disable midi out.

Set Drums to that only input controller it uses (Not None, not Omni), and uncheck "Transmit Midi Out" in the rack view. But yes, there is definitely a problem with it as described in my other thread. 

Yes, it just started happening to me. Super annoying!

Thanks to all for the tips and suggestions!  Will try them all and see what happens.  I never even thought of the "disable midi out" thingy.  Thanks!!
 

Y.O.P, I sometimes have the same problem...  The other suggestions made will probably work for you, especially  the"disable midi out," but even then I would think that if the input was set to "none," you should get none.  What I end up doing is selecting an input from my Focusrite, sounds weird, but it works. 
 
Jesse Q. Screed

"None" in Cakewalk's mind means "Omni."