Searching for multiple buried avalanche victims is an advanced skill. For most people, it is a better use of training time to focus on single burials.
When more than one person is buried in an avalanche, the searching transceiver receives multiple signals. If you have a limited number of rescuers (e.g., four or fewer), the best strategy is almost always to use your transceiver to find the closest victim, dig him up, turn off his transceiver, and then search for the next victim. With limited resources, and with the realization that more than half of avalanche victims are dead within 15 minutes, this strategy puts all your effort into quickly saving one victim rather than slowly attempting to save two. The exception is if the first victim is a deep burial (e.g., six feet or deeper) where the victim is unlikely to survive.
Avalanche transceivers direct you to the strongest signal, so searching for this person, turning off his transceiver, and then searching for the next person, can be thought of as performing multiple single-burial searches. This is an excellent strategy for most avalanche accidents (where resources, aka shovelers, are limited).
If you do have enough people to extricate multiple victims (i.e., more than enough shovelers to extricate the first victim), then you should locate the first victim with your transceiver, confirm it with a probe, and while other rescuers begin excavating the first victim, the remaining rescuer(s) can search for the next victim.
You will need to use special search techniques if you begin searching for additional victims before you are able to turn off the first victim's transceiver, because your transceiver will be receiving beeps from more than one transceiver. Consider two cars at an intersection with their turn signals on (and the adjacent illustration). If the cadence of the turn signals is not identical, there will be times when the lights of both cars are blinking at the same time, periods when they are partially overlapping, and periods when the lights are blinking independently. This also happens with two transceivers. There are times when both transceivers are transmitting a "beep" at the same time ("signal overlap"), times when they are partially overlapping, and times when the beeps do not overlap. Unfortunately, when the beeps overlap, the searching transceiver receives this as one beep. This makes it difficult for the searching avalanche transceiver to distinguish between multiple victims.
Some manufactures, such as Mammut, use a variety of transmission cadences in their avalanche transceivers. That reduces the likelihood that two transmitters will be continuously overlapped (e.g., the Barryvox and Barryvox S have 10 different cadences which vary based on their serial number).
Articles about avalanche rescue often claim that avalanches with multiple victims are rare, and everything is relative, but between 10% and 20% of avalanche accidents do involve multiple victims (I've worked two multiple burials—none of the victims were wearing transceivers).
If you have enough rescuers that you can begin searching for a second victim after finding the first victim (and before turning off his transceiver), you can use your transceiver's electronics to "mark" (i.e., ignore) the first victim's signal, or you can use the generic expanding circle or micro search strip techniques. The advantage of using the transceiver's "mark" function is it is fast; the disadvantages are the mark functions don't work consistently and some transceivers don't have a mark function. The advantage of using the generic techniques is they don't rely on complex electronics; the disadvantage is they require significantly more skill and practice.