A single frame can monitor multiple events or in this case, the same event for more than one unit.
Lua Code:
local function AuraCheck(self, event, ...)
local unit = ...
if unit == "target" then
print("Target Aura changed!")
elseif unit == "pet" then
print("Pet Aura changed!")
end
end
local f = CreateFrame("Frame")
f:RegisterUnitEvent("UNIT_AURA", "target", "pet")
f:SetScript("OnEvent", AuraCheck)
You can only register a maximum of two units with RegisterUnitEvent but if you need more, you could just use RegisterEvent and extend the unit checking in the event function.