Assuming you are using an ASIO driver, no. This is one of the very few "problems" inherent in this driver model, it allows only one input source to be used.
You may be able to use a different driver type, e.g. WDM, or WaveDriver, but this will inevitably introduce latency (= delay) which may, or may not, be a different problem for you, especially if you are using MM to overdub and existing recording.
The answer to this is to download a demo of FLStudio. This gives access to the FLStudio ASIO driver. This allows more programs than Music Maker to access the soundcard.