You aren't keeping the frame and texture components properly seperated. You only need one frame but two textures (no point creating a seperate frame for each texture).
Code:
local aggro = CreateFrame("Frame", "GrimUIArtAggro", UIParent)
aggro:SetPoint("BOTTOMLEFT")
aggro:SetPoint("BOTTOMRIGHT")
aggro:SetFrameStrata("LOW")
aggro:SetFrameLevel(2)
aggro:SetAlpha(0.5)
aggro:Hide()
aggro.L = aggro:CreateTexture("GrimUIArtAggroL", 'LOW')
aggro.L:SetPoint("BOTTOMLEFT")
aggro.L:SetTexture("Interface\\AddOns\\!GrimUI\\Art\\Aggro")
aggro.L:SetHeight(170)
aggro.L:SetWidth(285)
aggro.R = aggro:CreateTexture("GrimUIArtAggroR", 'LOW')
aggro.R:SetPoint("BOTTOMRIGHT")
aggro.R:SetTexture("Interface\\AddOns\\!GrimUI\\Art\\Aggro")
aggro.R:SetHeight(170)
aggro.R:SetWidth(285)
aggro.R:SetTexCoord(1, 0, 0, 1)
Edit: Bad cut-and-paste