Tandem and Twin are both similar, but in Twin Drifting, you're limited to 2 cars only, whereby Tandem usually applies on multiple cars.
These 2 forms of drifting belongs to exhibition drifting, the drifters are drifting in pairs/groups for the sake of excitement and fun only, and there's no passing or overtaking either, just doing it for show.
in D1, those drift battles are called "Tsuiso" (Chase and Run), its rather different since two competing cars are actually giving pressure to one another (to win), the drifts may not look clean or as smooth as tandem/twin, but its way more intense as the driver tends to keep his car extremely close to the leading car, forcing his opponent in making mistakes, and overtaking is common in Tsuiso.