Nope. Go for it!
You can blend wines any way you want. Mixing finished wines into new wines is a common thing.
Ideally you want to have your must set prior to fermentation. For many reasons, life doesn't always work out that neatly. Generally speaking, let a wine finish fermenting -- the volume doesn't matter until you move to a secondary container, at which point you want to avoid headspace.
Once the wines are done fermenting, mix 'em as you see fit. Winemaking is a win or lose proposition -- if YOU are happy with the result, you've won. If you're not happy with result? Cook with it. All wines taste alike after being boiled.