I don't like to mix brands because different brands have different additive packages, which may or may not be fully compatible. If they're not fully compatible, increased wear or sludge might result, but it wouldn't be enough for you to notice if you only ran it that way for less than 10K miles. Even if you ran Frankenoil for more than 10K you might never have an issue that you'd notice.
So can you mix oil brands? Yes, but it's not ideal.
Should you mix brands? Normally no, but if you're low on oil then adding another brand is better than being low on oil. A Frankenoil mixture is much better than being low on oil.
P.S. - If both oils are made by Warren, then it's the same oil. So you're not mixing brands.