This is a high fantasy trilogy that I read in the early 90’s where the magic system had a very unique requirement that required a person that was sensitive to magic power, who would dedicate his life not to casting/making magic but only with the ability to absorb the magic power around him like a battery and then was able to give that power to an actual mage that could then use it to make magic. The mage would be useless without having access to this other person who would feed them magic power.
Also by the third book the magical world gets invaded by a world/reality that has modern technology like tanks and guns.
I’m not convinced this is it, but the Chrestomanci series by Diana Wynne Jones does contain these elements – different worlds that can invade each other, some of which are more “modern” seeming than others – and there is a character, Tonino, who has the power to boost others’ magic in the way you describe.
Thanks for the suggestion but it looks like this series was published in 2013 and I read the trilogy back in I think 1992.
I found it!
Forging the Darksword: The Darksword Trilogy