How to separate the two dogs that are mating? Someone's chosen to force it.
Stop! It is normal for dogs to be separated during mating and to stand against their backs, known as Tie Phenomenon. During dog mating, locking can last 15 to 45 minutes.
A lock knot (mog)

We explain this magical phenomenon in two ways.
Why not?
Dogs have two special configurations of their genital organs: penis bones and turtle headballs.
A dog's penis bone. The arrow means a urinal. Trench
Dogs do not get into their vagina with their penis erections, but with the support of their penis bones, and when they are stimulated by their vagina, their headballs expand rapidly, while their vagina shrinks and “locks” their penis, at which point the penis cannot be removed. The longer duration is mainly due to the fact that dogs do not ejaculate during mating, but are slowly delivering semen into the vagina of the mother.
I can't explain myself.
Why back to back?
This question has nothing to do with pregnancy. It has been argued that in the field, dogs are also required to detect the environment behind the mating, and back-to-back helps to maintain vigilance. But I personally think it's only because dogs are tired on two legs (equivalent to a single leg), and on the other hand, they're tired on the other side, so they can stand on the other side of their backs, and it's easier.

Who? Who's gonna attack me?
But the most amazing is more than that!
I think the most amazing thing is that we two-legged animals know these principles, but the Wang don't! In other words, dogs only know that they're attracted to them because of their reproductive instincts, and that they're attracted to them because of their instincts, but they don't know the reason why they can't be separated once the mating begins (until their penis is reduced). What's the consequence of not being in a position? It is the cases of male dogs and female dogs who feel uncomfortable, nervous, anxious, and even the cases of female dogs who bite their dogs because of their reproductive tract pain.
When the wolf mates, there's a lock-down of anxiety.
In the field, mating is dangerous.
So what can we do?
It's normal for you to see a dog nervous, but remember - don't try to separate them! Dogs who are blindly separated from their mates suffer severe pain and both are likely to be injured. In fact, a relatively safe and quiet environment for dogs is enough. If you're a more experienced pooper, or if you know that your dog is gentle and has a good relationship with you, you can try to appease the dog and distract her. But it must be safe to do so, and the severe pain of the dog could cause it to lose its sense of reason.

Don't panic. Just hold on.
Speaking of which, I can't help but think of Meow's mate, which is worse than Wang's! Wang can only cause pain, and Meow is bound to cause severe pain. It's mainly because the male cat's "breathing horns" are stingy, stimulating the female cat's ovulation by scratching her vagina's walls...
Please have a sense of self as a light bulb.