Label me ignorant, and then please explain why formation flight mandates, guarantees, and requires an engine failure. "Not IF it quits. WHEN."
ALL single engine flight does.
Formation flight is going to take a significant portion of your attention away from things, especially if you're not the lead aircraft.
Lead shouldn't put wingmen in the situation where they only have 1000' to find a spot and land it WHEN their wingman's engine quits. Wing shouldn't accept that as the plan for the flight either. Not carrying passengers, especially.
Nobody has leaked whether or not it was just "loose formation"/flight of two type stuff, or serious formation though.
But formation should be briefed, and if someone said "1000 AGL" out there in any briefing I'm in, I'd be asking for 2000' AGL -- no reason not to, out there.
Maybe they were so heavy they were still in the climb. Dunno. (Shrug.) Seems like they could have been higher coming out of any of the COS area airports by the time they got out that far east.
I'll admit to a bias here also. Early in my flying two well known pilots took their airplane "out there on the plains" and looped it. Straight into the ground on a hot day. There's a bit of a siren call to Denver metro area pilots to go do stupid stuff over the plains, since they feel free to choose to go low level away from the front range airspace and "rules". It's been an underlying factor in a few fatals over the last 20 years that isn't mentioned much, since it's one of those impossible to prove things.
But there's not much reason to be low out there. Doing air work, formation flights, aerobatics, whatever. Give yourself some time to look around and there's all sorts of stuff to land on/in out there. It's the high desert until you go downhill significantly toward Kansas or Nebraska. 3000' AGL is required out there to do air work in the twin and on checkrides... And it has a lot more options than a single...
We've all done "not always the most conservative option" things out there. I'm just saying it sometimes leads to realizing low level out there isn't the "best" plan overall. You have to be on your game WHEN the engine quits at 1000' AGL out there. Not much time to deal with it. 2000'-3000' AGL gives you another few minutes to figure out where you're going.
A favorite of CFIs around here is to go do air work out there at 2000-3000' AGL and then pull your engine directly over one of the private ranch strips in good condition out there. About 50% of students line up on a pasture instead, CFI friends tell me. Heh. That one is a way to point out a lack of SA when maneuvering. Really was a tough one when we all used paper charts. A moving map makes that little "test" a lot easier. I've gone out and taken a closer look at all of the ranch strips south of my house -- out of about five charted, three are in great shape. One even has a windsock and is obviously used regularly by the owner.