Until now, my bent has been slightly away from double Combined Arms Detachment legality, and slightly toward restricting to just one.
I'm going to try to challenge the community to tell me why 2 Combined Arms Detachments are any worse or better than the status quo.
Let's take a look here ...
First off, there are powerful lists available in 40K no matter what you do, short of over the top comp and rules rewriting. Even then, there would be new powerful lists, you just might make more codices capable of them.
But let's look at what people can do with it from a broad sense and see if any of it is really all that much worse or better than the status quo, while acknowledging that it broadens the meta of what you have to consider regardless:
6 Annihilation Barges, which to field requires a minimum investment of a cool 980 points (2 naked overlords, 4 naked walking warrior squads, 6 barges would be 980). Yeah, it shoots really well, about like a standard single CAD of 6 Wave Serpents does.
What about 4 warscythe 2+ armor MSS lord catacomb command barges?! Well, they have precisely 3 attacks + the MSS after the charge, can be locked in combat, the lord can be targeted and so can the barge once in CC, and cost 210 points a pop. Yes, they're much better, and no they aren't that good still. Unless they roll high on the charge, they average less than 2 marines killed in a round of combat, which means even 5 model combat squadded marine squads can hold them up and give them trouble (or even krak grenade hp away). So if you added those in to the above 980 for trying to get 6 barges, and wanted to spam 4 ccb, you'd be at 1640 with your objective secured / troop units being a grand total of 4 x 5 walking warriors.
Space Marines / Dark Angels
Obsec Drop Pod spam gets a little worse, and a little more focused ... though there's a limit to how dramatic and how effective that can actually be.
There are two concerns here ... one being getting access to more objective secured wave serpents, and the other being access to more farseers for the council.
Well, let's look at both ...
The only serpent advantage you get is 20 points less if you're trying to spam serpents after the first 6 (you can currently gain outflanking/infiltrating serpents for a 20 point premium on top of the avenger ones by using striking scorpions), and obsec on a few more serpents. So you're largely prevented from gaining MORE serpents, but the extra ones you get (going to normally cap at 3, at most, more) ... in fact even with just avengers, if you're taking ghostwalk matrices (and you've got more problems as a player in a terrain heavy environment than your list if you don't) and a pair of 70 point min costed HQ, you're looking at 8 total serpents ... and that's not a great list beyond the fact that it's serpents and they shoot pretty good / move around for objectives pretty good.
So let's look at the council idea. If you want to skip on 45 points of allied troops and 105 points of hit and run / stealth by not taking DE as your 2nd detachment, and want to take a second CAD instead, you're looking at a couple of things:
First, your troop tax went from 45 to 102. Second, you're probably taking at least one farseer (115 instead of 105). Third, you've lost stealth and hit and run. So if you want hit and run back, you literally have to spend 195 on Baharroth. If all you want is hit and run and not a third psyker, well, you aren't going to use the 2nd detachment anyway. Finally, if you do NOT take hit and run, and instead try some nonsense like 2 councils, you're going to lose the moment you run into an ork or tyranid player. Tough luck.
Someone brought up 6 Wraithknights ... which certain armies crap all over (looking back at tyranids, venomspam, etc.)
Alright, how about Tau?
Tau can spam 18 broadsides this way, but they can get pretty close to that anyway with the Fire Support Cadre, and in that situation 6 of the Broadsides are going to be much harder hitting. Tau could also throw out 6 riptides and/or 6 skyrays, but are we really that concerned about these things? Riptides without buffmanders and markerspam do not hit very hard and still are non-fearless low skill MCs just begging to draw any kind of assault army for the loss. You already can see 4, and with far better support, in a more balanced Tau list.
OK ... let's keep going ...
8 Level 3 Tzeentch Heralds turning into a bunch of Lords of Change!
If it hasn't been clear yet, the Daemon Summoning Factory list is not very good. Furthermore, it is extremely risky to take to a tournament. Here's a sidebar for you ...
If you show up to the NOVA Open with a Summoning Factory, and you throw down a bunch of identical-to-your-units model sets that can't be easily distinguished from your own models and create extreme difficulties for your opponent, you are at high risk of having those models removed from play by the organizers. You're also going to have your own struggles just placing and moving and keeping track of everything. Plus you're going to struggle on mission. If you bring a summoning factory, every model you place down must be fully painted, WYSIWYG, and clearly distinguishable from other units in your army (just as your own army should be). Don't push your luck, you aren't going to get a lot of freedom of "oh it's fine you can roughly tell" with this.
6 dog-medicine-paper-coned helturkeys? Err ...
I mean, I could keep going ... but I don't know how any of this is any worse or more painful than "I have 8 wave serpents, 6 of them are objective secured and 2 of them outflank or infiltrate!" or "I have a big beast pack with invisibility, fortune, hit and run, and I can't get all those powers off every turn but whatever it's scary!" or "I have a jetbike seer council with hit and run and fortune/invis!" or "I have 3 Imperial Knights and a bunch of MSU AM in support!" etc. etc. etc.