No, Carlisle, I don't think you'll lose Mackie mode - that's the point, if it works, of the two different midi ports - you set them up as two different controllers. Basically what you do is go to the control surface dialog and add another controller, as you did already for mackie, but using the external midi port of the project Mix instead of the control surface port. Also, you do not choose mackie from that dropdown, rather one of the ACT choices - generic ACT controller, or something like that. There are two possibilities, if I recall, related to ACT on dropdown, it's one of them(I'm still not at my DAW, so I can't confirm accurate specifics).
I know that's probably not enough info, but it's the basic concept we're after. I have to go do something else right now, but if you'll bear with me, I'll try to find and post a link or two to some Cake instructions or tutorials that got me on the right road.
Again the idea is making the projectMix appear as two logical controllers in Sonar, one on each midi port - a MCU surface, running through the Mackie surface plugin internal to Sonar, and a generic ACT surface, running through ACT. There's a bunch of ACT setup, but it's straightforward once you get it.
Here's the catch - the knobs, etc, that you want to use with ACT have to be transmitting their midi values on the midi external port. So we'll have to determine how to make the Project Mix do this. Ideally it will be as simple as a button on the unit that switches it from one controller mode to the other, that's how (sort of) the novation works (the button switches templates, so it can have a large number of different 'surfaces' configured for various things, but that's beside the point).
anyway, I'll try to get you a link up later on. Will you be back on this evening? also, if you have a link to the manual for your gear would you post it please?