This should be an easy identification, but my google-fu is failing.
Late 70's/early 80's. Two brothers. One a magic user, the other a technologist. They fight a war, I don't remember who wins. Hopefully I'm not conflating two novels here, but the magician sees magic as threads, which can be woven and unwoven to make 'spells' or powers.
I'm fairly certain that the technologist had tanks and robots.
Should be one of the better known sci-fi/fantasy authors.
I think this is probably Roger Zelazny's Changeling (1980).
Two young boys are switched between a magical world and a technological one, and grow up favouring the system of their birthworlds, making them outcasts. Pol, the magical one, sees magic as strands. Quoting from the plot summary on Wikipedia:
In the world of magic, the young Mark Marakson is obsessed with devices, building water wheels and later, steam engines. He does not understand why the people on the farms and villages rely on magicians rather than using the machinery he creates. Young Pol, meanwhile, grows up a poet, musician and singer, marked by the white streak in his dark hair. He is a great disappointment to the man he regards as his father, who is an engineer. From time to time he sees glowing strands in the air which he can touch to make things happen.
Mark is ostracised by the people around him and wanders in the hills until he finds a graveyard of machines, left from the ancient war between magic and technology. Able to restart them, he returns in triumph on a flying machine to claim his childhood sweetheart in the village, only to be assaulted by the villagers, losing an eye. Fleeing back to the graveyard he creates an army of machines to take revenge.
Mor, realizing that he has disturbed the balance of the world, goes to retrieve Pol to counter Mark, revealing Pol's heritage and powers to him. To return Pol, the elderly Mor has to remain in the technology world to balance out the transfer. He dies in a park where every tree, bird and insect is artificial.
There is a sequel Madwand (1981), but it's mostly about Pol finding his place in the magical world. (And a sentient curse that likes the colour green.) If you recall the name "Belphanior" you might have read the 1989 omnibus Wizard World which contains both novels.
I know the timing isn't correct, but reminds me of Urza vs Mishra in the Magic: The Gathering novel The Brothers' War. Urza is a planeswalker (magic user) and Mishra primarily uses technology. The whole book revolves around the two brothers who would rather right than forgive. It's not necessarily from a better known author but it's part of the Magic: The Gathering series which is relatively well known.
This also reminds me of the Darksword Trilogy from Margaret Weis and Tracy Hickman from 87-88.
However it is not brothers. But one person, the son of the Emperor, is born without magic in a world where everyone has to have magic. He is cast out (sent to a different dimension) thinking it means his death.
However the other dimension is technology based and he comes back to his original world to wage war with guns and tanks.
Not sure this is your answer but it might be mixed in with another book of the time frame.
If you are talking about a movie, then "Wizards" from 1977 by Ralph Bakshi is what you looking for.
3,000 years later, Blackwolf has risen to lead the dark land of Scortch, where he and his vast army of goblins, ogres, wraiths, demons, dragons, and mutants salvage and restore ancient technology. He tries to attack Montagar twice, but is foiled both times when his mutant warriors become bored or sidetracked in the midst of battle. Blackwolf then discovers an old projector and reels of Nazi propaganda footage, using his magic to enhance it for psychological warfare: inspiring his own soldiers while horrifying enemy troops into submission.
