How to define SNR for multiple signals in noise?
The measurement that is important to you will depend on the application. If you are looking for an over all measurement of all signal power to noise power then you define signal to be the power in all signal bands and the noise all of the powers in the noise bands.

However, if you are trying to use an sub-band adaptive filter to correct for some sort of distortion then the SNR in the particular band that you care about would be important not an overall SNR.

In the same application both measurements could be important at different stages of the system. If you are working in wireless communications and the frequency range that is being digitized contains multiple signals then the over all SNR into the receiver needs to be high enough to limit the noise introduced by amplifiers before digitization. But if after I digitize the band it is then split into multiple sub sections for demodulating the signals I would only care about the SNR of the signal currently being demodulated.

For your application it sounds like an over all measure is important because you are treating the range of frequencies as a whole.

In the application of mismatch correction algorithms for time-interleaved ADCs (my topic) we sometimes use multiple sinusoids input to the interleaved converters to measure the performance increase the correction algorithms. This makes it easier to calculate and visualize things like SNR and SFDR when the desired signal locations are known.

I hope this helps,

Charna