I think Talysman's rule is too fiddly (no offense!) and it occurred to me that there's already a common "house rule" that could be extended instead: lots of people seem to give characters who attack with two weapons (aka "dual wielding") a +1 on their to-hit roll. (I don't want to debate whether that's a "realistic" approach, let's just take the mechanic at face value.)
A direct application of this to the carrion crawler may not be such a good idea: they'd get +7 on their to-hit roll and that seems way too much of an advantage. (Actually it may not be, but I didn't want to run the probabilities and just went with "gut feeling" instead.) But we can go with an "exponential" system to "soften the blow" as it were:
Seems fair to me, and the math is nice too because the bonus is just the logarithm (base 2) of the number of attacks. (I should get extra credit for that!) But some may complain that this way ghouls with their 3 attacks are not scary enough anymore: a measly +1? Tweaking the cutoffs a bit differently we get this:
The math is now off, but hey, your beloved ghouls stay relatively scary while at the same time carrion crawlers are not completely "off-the-charts" either. And I don't even know of a monster with 9+ physical attacks, so the +4 probably never applies at all? Anyway, just my $0.02!
Edit (aka Actually Doing the Depressing Math): Carrion crawlers have 3+1 hit-dice and 8 attacks in B/X. Let's say one of these beasties is fighting a guy with AC 3 (plate mail) and no other bonuses, so it needs a 13+ to hit. Rolling 13+ on a d20 means a 40% chance to hit. Using (hopefully correctly) the binomial distribution we find that the chance for no hit in 8 attacks is only 1.6% meaning the chance for at least one hit is 98.4%! (The most likely outcome is actually 3 hits which has a 28% chance.)
That's rather sobering and sort of what I feared when I said "it may not be" in parenthesis above: Using the +7 suggestion the carrion crawler would make a single attack roll for which it needs 6+ on a d20 which is a 75% chance, a far cry from the 98.4% of actually rolling 8 attacks. It really should get +12 bonus, then it would have at least a 95% chance to hit.
Now let's do this again for a ghoul, just to see what's what. Ghouls have 2 hit-dice and 3 attacks in B/X. Again assuming an opponent with AC 3 a ghoul needs a 15+ to hit, that's a 30% chance. (This is actually small enough to run a simulation to double-check our math.) The formula comes out to 32.4% for no hit, meaning 67.6% for at least one hit. Translating that back into a attack bonus, we should give a ghoul +7 or +8 for its attack routine.
And what about a character using two weapons? Assuming we use the same reasoning, that there are really two attacks but we abstract that into a bonus for a single die roll, what should that bonus be? Let's say a level 3 fighter attacks a carrion crawler with two weapons. The carrion crawler has AC 7 so our fighter needs a 12+ to hit, that's a 45% chance. (Here's the simulation.) That's a 30.2% chance for no hit and a 69.8% chance for at least one hit. So instead of a +1, the character should get a +4 or +5 attack bonus for attacking with two weapons.
In summary, all these bonuses are seriously flawed approximations. I'd contend that you can still use them, but you should be aware that you're paying for the convenience of rolling fewer dice with (overall) much lower chances for a successful attack. Or the other way around (for the min-maxers out there): If you can get an actual second attack, you should almost always take it instead of taking a constant bonus.