The red thirst gives furious charge. The Sang Priest gives furious charge. If you have furious charge you get +1 Str and +1 Init. Had it said: The red thirst gives the unit +1 Str and +1 Init. The Sang Priest gives the unit +1 Str and +1 Init. then they would stack, but they both give furious charge, so we look at the FC USR and find we get gives the unit +1 Str and +1 Init.
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