Sour beer plus not sour beer equals sour beer (for the most part). If the brett beer is fully attenuated, and it probably is, then it probably wouldn't get any more sour since there's nothing for the bacteria to consume. Adding new sugar is going to give the bacteria something to consume and produce acids with. So the beer will probably get more sour. The brown ale added to the sour carboy will add a lot of sugar for the brett and bacteria in the lambic blend to consume. I'm not saying any of those plans are necessarily bad but you're not going to end up with a sweet-sour blend, if that is what you were going for.
Since the cherries will add only simple sugars, you'll want to give it at least a couple months to dry out. It would benefit from extended aging. The brown ale will need at least six months but for full flavor development, probably 9-12 months or more. Taste and stable gravity (and to a lesser extent, texture) will guide when it is ready. Bottling too soon is likely to create bombs or at least gushers. No reason to turn beer into a suicide device or watch months of aging gush out on your counter.