I do not know of any signal processing technique to separate one bearing vibrations from another. The common way is to have one or more measurement points on each bearing housing and use amplitude (peak value or spectral peaks) and frequency spectrum (matched to fault frequencies). Note that "envelope" is only one method of bearing fault detection. Other methods include velocity spectrum, high frequency acceleration and ultrasound (contact sensor) measurements. If bearings are close together, then a defective bearing and nearby bearing would typically be changed at same time. It is generally not wise to change a bearing based solely on envelope spectrum fault detection, unless the very early warning provides the best opportunity.