Long post warning
Yes sadly worked in a rescue where a lot of dogs came in as a littermate situation following fights and injuries
It's worth saying that a lot of it is about the complications of simply having two untrained dogs together rather than as something specific that happens with two dogs That are related. I'd also highlight that when you ask for advice a lot of people will tell you that their dogs are fine when their dogs are under the age when it usually presents (I'd say by 4 it tends to be more clear), or when their dogs clearly exhibit signs (inseparable, co dependent behaviours, can't settle alone and have behaviours that are made worse by being together)
I'm not saying all of the below is true for you but simply the patterns we see.
So in terms of having two dogs... do you have time? As in do you physically have the time, space and money to do the puppy thing doubled? Lots of people get 2 because they hope they will entertain each other, so may not even have Time for one.
Two dogs means things like space for 2 crates etc. They will need their own space at times and to get used to being able to settle apart (else it will come back to bite you when one goes to the vets, or if they need to be separated due to injury, fighting etc). All young puppies lack an off switch, so you'll have to manually manage that. The same as when they are teenagers, they will take great delight in winding each other up when the other wants to sleep. Typically if you have an older dog, the younger pup will listen to their no but it's harder with two teenage terrors winding each other up
Both dogs will need independently have training space so its double the training time . It might then be than you need someone needs to distract the other pup while you're working with the other otherwise they disrupt each other all the time. Lots of people struggle to have the time, money and man power to put two pups through puppy training classes at the same time. Often you'll need a second person so you have a 1:1 ratio.
The best example I can give is public walking. With one puppy you can clearly see when they are fearful, might need a different approach, space and it's easier to anticipate behaviour so you can tell for example they are going to pounce on another dog so redirect their attention. You can mark and reward clearly
With two dogs with the same handler you lose that focus, so you can tend to yank them into situations they aren't comfortable with, or let them practice undesirable behaviour behaviour because you don't have the hands to sort it. This is more likely to lead into having a reactive dog. If you are trying to teach lose lead walking you might be saying yes to one while it's walking beautifully, but their sibling that's pulling manically will also hear that same yes. You'll also be missing things to reward and doing more pulling on leads accidently which encourages dogs to accept pressure on their lead as normal eg when dog a is walking lovely, but dog b pulls, you'll pull dog a too.
What then happens is it becomes a bit of a circus, they hype each other up so say for example one barks, the other will copy that bark some more and they will hype each other into a barking crescendo. Its why it's often people with multiple dogs that have barky dogs on walks
Obviously what then happens is that people decide that it's a bit overwhelming and tend to do less socialisation of their puppies, and they get less socialistion than the average pup. Imagine trying to introduce two pups to a horse for example at the same time. It's hard work!
None of that is insurmountable. For example we often do a one person one dog approach with my partner when walking young pups and it works perfectly. Infact we do it now as we have an old dog that wants to sniff the flowers and a young dog who needs a bit more of a pace. We walk together just using different techniques
If you have the space/time to train them individually and give them the same attention, time out etc you'd give a single dog then you're off to a better start. Just like human twins, they need time to be their own dog, with their own personalities that might need different things from you
However if you aren't sure then they'd be better of a singles.
Most reputable rescues will be cautious of it, and to be honest I'd take it a giant red flag if they seem keen for you to do it. Especially as a first time fosterer