Tandem feeding is the twin-specific art of feeding both babies at the same time instead of one after the other. On the breast it usually means nursing both together, supported by a twin feeding pillow. With bottles it means feeding both at once in a propped, supported position.
The reason parents reach for it is time. Feeding two babies separately can swallow the entire gap before the next feed, whereas tandem feeding turns two jobs into one and keeps both twins on the same clock.
It takes a little practice to set up, and it is not the only way; many parents tandem feed at busy times and feed one at a time when they want calmer, one-to-one feeds. Whichever you do, stay with both babies throughout and never prop a bottle and leave.