I have an Old Ale that is too sweet. I’m thinking about brewing a pale bitter and while it is fermenting blending some of the Old Ale in part because I think there are still some fermentables in the Old Ale. When I add some Old Ale to a fully fermented Scottish Ale made by concentrating some of the first runnings, the Old Ale completely takes over the flavor. If I referment the Old Ale with the pale bitter will the refermentation reduce the aged flavors of the Old Ale or is the Old Ale likely to take over again (althought the pale bitter will have some balancing bitterness)?
I don’t know for sure, but blending does solve some of the issues/problems from under attenuation, since the yeast has another chance at it - you have to useable strain that is a strong fermenter, given the alcohol level. Give it a shot. I have some Barley wine that I blend, just to get the cloying flavor down for drinkability and it works. Straight up it tastes like a barley brandy, but when blended with a lighter beer like a bitter, the blend is quite palatable.
You won’t lose the aged flavor of the old ale although dropping the old ale in during fermentation might blow off some of the aroma from the old ale. I’d probably look at adding the old ale right before primary fermentation on the bitter winds down in the same way people will add sugar additions to a tripel as fermentation starts to slow down. Avoid some of that very aggressive fermentation in the beginning.
Good points. Glad to know that I won’t loose the aged flavor which I enjoy. I just need to reduce the cloying sweetness and balance it better with bitterness. Also I want to lower the overall alcohol level.
Before going to the trouble, I’d do a fast ferment test to determine that you actually have some fermentables left in the Old Ale. If there aren’t, you can just blend the finished beers to taste.
Is the fast ferment test basically adding a crap load of yeast to see if the gravity changes?
that’s the one
But just to a sample…not the whole batch! 
Pretty much