I was wondering if it makes much different to use two different brands of paint (of the same color and sheen) when painting two coats. For example, if I have some Behr in semi-gloss left and it is enough for one coat and then buy more of Glidden, also semi-gloss, because it is cheaper, to do the second coat.
On the face of it, that's not a bad idea.
I would only be worried about a few things, though:
One, is about the actual sheen that each brand presents. You really want to try both out on a scrap or hidden wall and make sure the brand you want to have show up on top is carefully selected.
Two, is about how easy will these varying coats be, for patch repairs as life happens.
Three, you generally don't double-coat your cut-ins. For example corners, around trimwork, etc., are mostly just a single coat (and you would roll as close to edges as possible), and doing two separate brands' coats on the entire wall surface will take a lot of (possibly thankless) work.
It absolutely matters. The paints aren't identical. If it's a home you're looking to do the right thing with, use the better paint for both coats. All companies have their high and low-quality lines, so don't just assume that all paints under one brand are better than another brand. There are a lot of good ones, including Glidden, Sherwin-Williams, and Benjamin Moore, but even they have lower-end lines of product.
As for the comment regarding not cutting the walls in for the second coat, that's a terrible idea. ALWAYS cut the walls in for the second coat. The brushed-on paint goes on thinner than the rolled on paint, so those edges usually need that second coat. Also, if your paint is anything but flat, those unpainted, dry edges will likely be visible after the walls dry. Don't do shortcuts.