EMG is actually used to detect subvocal muscles and inner speech which happens by minuscule movements in the larynx & it records the electrical activity, produced by the articulatory muscles. So in order to get such EMG signals, a careful placement electrodes on crycothyroid muscles is required.
But also in subvocalization different parts of the brain are being used, which might result in different EEG signals. So you can combine both Muscle SpikerBox & H&B SpikerBox in one experiment/study.