I can't tell for sure what killed your two tiarellas. Both lack of water and soggy soils can be deadly to plants. The fact that the leaves were looking scorched may point more toward dry soil or burn from an excessive fertilizer application. You can provide moist but not soggy soil for the plants and give them another week or so to see if any resprouting will occur, perhaps low on the main stems. If sprouting doesn't show up in another week or two they are not going to make it.
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