Mixing two oil wieghts

My Toyota minivan calls for 0w-20 motor oil. I have one gallon 0f 05-20 synthetic oil and one qt of 0w-20 synthetic oil. I believe the crankcase holds about 6 qts. Since I won't be travelling in extremely cold temps, I think I'd be OK with mixing the 2 types. I'm not sure if the mistake is mine or the vendor's. I don't have time to exchange the 05w-20 oil for 0-20. Any advice is appreciated. Thanks