Using multiple sources for the same profile will NOT degrade accuracy, BUT...
Assuming you are plugging both mics into the same USB sound card then you won't really be able to set them up as separate sources in the same profile. As I see it, you have two options:
1: Use both mics with the same profile, and simply run the mic check each time you switch to get the best volume settings for that dictation session
2: Since you're using Professional you can export/import all of your custom commands and vocabulary and create a second profile for the alternate mic.
A third option would be to get a second, different, USB sound card, such as the SpeechWare MultiMatic which might improve the performance of the Sylencer thanks to the Auto Gain Control, and then you could set it up as a separate source in your existing profile.
See the Nuance Knowledgebase ID 6276 for more info.
Please update with what you decide to do and how it goes.