Depends on the grenade... Mostly unlikely as hell, but possible, in a critical glitch kind of way. I know this got solved in 4e by War!, so while I don't know the wording in 5e book, here's the bit from 4e...
Occasionally, a gamemaster will have more than one grenade or other explosive simultaneously affecting a single target. In these cases, treat the damage as a single blast with a Damage Values equal to the sum of the highest Damage Value of one explosion plus half the DV of the other explosions. Each explosion’s DV should be modified for the distance of each explosion, as if the target was affected by the Blast in a Confined Space rules (p. 156, SR4A). The combination of overlapping blasts and confined quarters is known as the “puréed salsa effect.”
Dunno. Even the bits I've seen here, it seems pretty cut and dry to me. Multiple grenades work as multiple explosions, unless you're just scattering them all over the place, but at a single target or a group, it's going to be very very painful for them.