I would suggest fermenting the two separate and combining them after the fermentation is finished. The yeasts are chosen for these fruits for a reason. I personally don't know the reason, but the fruits are very different. Also, using two yeasts and blending after fermentation is finished will add to the complexity.
The dark place is helpful, but you can cover your fermenters with T-shirts or bags.
The chemicals are there for a reason. Most likely yeast health. I would follow the directions on the kits and get some experience with what does what before you start changing things.
Assistant Winemaker/Cellar Manager
Inwood Estates Vineyards and Winery
1350 Manufacturing St. #209
Dallas, TX 75207