I get a different number of samples than you. After the Ideal Mux the Matlab component contains a sampled signal of 2097152 and before each sampled signal is 1048576. The output of the Ideal Mux is then twice the number of samples of the input, yet containing 5 channels.
The sampled signal is twice the original the sample rate of the individual channels because although there is some overlap of the signals the resulting signal needs to represent a larger bandwidth signal and thus needs to have a larger sample rate to span the spectrum.
The sampled signal is centered at the average between the input wavelengths and contains all of the sampled information.
Regards