Just for the record; by RAW, it stacks. (I dont know if it auto-hits though). I think this might work great. Two of those big critters are pretty cheap, and they will have greatswords hitting on 2s and wounding on 2s against standard infantry (with re-roll from warrior priests) *drool*
I will repeat this, it will not auto-hit. BRB pg. 50, Paragraph starts with "If you roll too low...", the paragraph goes onto say (following is not word for word) 'a natural roll of 1 will always fail'. Stacking at the moment is okay, although I wouldn't expect every opponent to be 100% happy with this, but auto-hits are out.
As for tactics, personally I think a better use for two would be a wider range of +1's instead of a short range of +2's.