Newborns School Nurslings!
I'd offer you a phrase or two, but I'm afraid it would be a clear case of a putz/simpleton/jackass/buffoon/bozo advising/counseling/instructing/coaching/informing a fool/idiot/blockhead/ninny/dimwit. Give a man a proverb, after all, and it may vanish with the dropping of the first veil of dementia. Encourage a man to compose his own proverbs, though, and you nurture a lifelong habit of self-indulgence.
Choose your terms sensibly. If you're an Australian, the first may be a wally, the second may be a nitwit. Then stitch them together with the right verb: that wally will knock some sense into the nitwit, for sure.
Or keep it simple by calling both the first and second fools, and have one do for the other whichever of the many things it is fools don't do well. Keep it varied by choosing the terms (noun-verb-noun) to slot into the formula depending on the circumstances. It's a game you can play well, I'm sure.
Chimp Sets Table For Barbarian!
The origin of the "blind leading the blind" proverb may be this:
Abiding in the midst of ignorance, thinking themselves wise and learned, fools go aimlessly hither and thither, like blind led by the blind.
(From Juan Mascaró (tr), The Upanishads, Penguin Classics, 1965, p. 58.)
In various metaphorical forms, the proverb also appears in the Canki Sutta (an article of the Buddhist Pali Canon), Horace, the Gospel of Thomas, and Matthew and Luke of the Holy Bible.
These distinguished and varied origins suggest, and Dr. Betsy Zaborowski, (former Executive Director, National Federation of the Blind Jernigan Institute) confirms, at least for herself, that you needn't concern yourself much about pushback when you use "the blind" in the way it's used in the proverb.
Goths Etch Miniatures for Vandals!
However, you asked for an alternative "that won't be offensive to anyone". I think not being offensive is situational: if you're among friends, you know (you hope) what will offend them and what will not. If you're not among friends, you know at least what is likely to offend and what is not, depending on your audience. The choice, barring ignorance, of whether or not to offend is yours, and how you go about avoiding offense is beyond my purview. Alternatives, though, those can be produced in abundance, and the likelihood of them being offensive can be assessed (by you) by examining the terms used.
A case of the unlearned teaching the unschooled.
A case of an ignorant instructing an ignoramus.
A case of the cart trying to pull the horse.
The first and the last of these seem least likely to offend. The second...it depends on the sensitivities of those involved. Of the first, it seems fairly neutral: if said with reference to yourself and another, you've at least insulted yourself as much or more than you've insulted that other.
The third (which incorporates an improvement from the OP's comments on this answer) borrows from the familiarity of another proverb ('putting the cart before the horse') and intertwines the two proverbs: the horse pushing the cart or pulling it backward is preposterous, and the cart pulling the horse or pushing it backward is preposterous. Voila! A potentially offensive proverb has been replaced and the replacement does double duty as both old and the new: a horse not capable of pushing and a cart not capable of pulling are both in the position of doing just those things, pushing and pulling, with each other as both the leader and the led.
Of the three explicit examples, for your purposes, I favor the first, mostly because the third is a braintwister. Beyond all the examples, though, it's notable that nobody (or very few) think that they are fools, dimwits, or simpletons, and so are unlikely to be offended by being compared to one in the same breath that you compare yourself to one--you can't be serious calling yourself a fool, and therefore you are not serious calling them one.