A single attacker could have restrained her alone, especially a strong male attacker.
There is NO evidence of two knives. There is evidence that the kitchen knife could not have created anything but the slash while not using full force. The kitchen knife was not the murder weapon.
Well, there
is "evidence" of two knives if (and only if) one accepts that the kitchen knife was one of the murder weapons. And of course this is because the kitchen knife
cannot have caused all the neck wounds. Thus, if one believes the kitchen knife was one of the weapons, it's axiomatic that one must also believe that two (at least) knives were used in the attack.
However, the wounds themselves are entirely consistent with having been inflicted by just one narrow-bladed non-serrated knife (with only one sharp edge) with a blade length of around 8-10cm. And this also correlates with the faint outline of a blade that was made in blood on the bedsheet.
So if the kitchen knife gets thrown out as evidence (which I believe it will), there will be absolutely NO evidence whatsoever that more than one knife was used in the attack. And if that's the case, it's far more logical to conclude that the wounds were all made by just one knife, wielded by just one person (regardless of - and separate to - the question of whether others might have been involved in restraining Meredith).
To use an analogy, imagine that someone is killed by being hit over the head twice with a hammer, leaving two circular indentations of similar size on the victim's skull. Now, it's entirely possible that two different people perpetrated the attack, each of whom was armed with a similar-sized hammer, and each of whom struck the victim in the head*. But, in the absence of any other evidence pointing at more than one assailant, the default conclusion would be that the two blows were struck by one assailant using one hammer**.
* And, if asked directly, I suspect that any forensic pathologist would agree under questioning that the two head wounds were
consistent with having been inflicted by two separate attackers, each armed with similar hammers.
** And of course a pathologist would also agree that the two head wounds were also
consistent with having been inflicted by a single attacker wielding a single hammer.