In regards to this question I've been inspired to take my flat beer and blend it with a new beer. My main question is when is the best time to combine the two beers? Should it be done at the start of primary fermentation or much later? Are there any "gotchas" or non-obvious things to keep in mind during this process?
In the past I have blended at Packaging time (for me this means kegging). This has had good results for me in the past. This also will ensure that you when you blend you can blend to taste versus blending blindly and not knowing what you'll end up with. I believe this matches major commercial practices as well.