So in my mind, if a sniper rolls Sniperist and Thief, the 2nd class is infiltrator, i.e. big damage sniper. If a sniper has Sniperist but lacks Thief, the 2nd class is assault, i.e. cleanup sniper. Then there is this guy:
Thief would point to dual class as infiltrator but, on the other hand, this could also be a very versatile assault. An an assault, judicious use of rapid clearance, quick aim and different types of weapons (ranging from 1 to 3 AP per use) could allow the soldier to convert will into AP, creating something like a terminator build, although one that requires more support from circumstances or the rest of the squad.
So if this was your soldier, what would you multiclass into?