I remember back during the 94 AWB that welding mag bodies together to get a longer (higher capacity) mag was a thing.  Is it still a thing and is there anyone around that still can do it?  I would like to have a couple of steel mags modified, who can I talk to?

Still a thing, as in can it still physically be done? Sure. No legal advantage to it anywhere that I know of at the moment, but if you want to do it for kicks and grins, then have at it if your state or local laws don't already forbid anything over a certain capacity outright. You don't list a location other than "USA", so I can't be more specific than that.

As for who you can talk to - anyone who can weld thinnish sheet steel, like a TIG welder or welding shop that specializes in TIG, could probably trim and join up those mag bodies for you.

Biggest problem is avoiding making a bead on the inside that will catch the magazine follower.
Warping can also be a problem.

I am smack in the middle of Illinois, so no ban or anything.  What I have is 30 round Grease gun mags that I use in PCC.  I ONLY have 30 round mags, what I was thinking about was to cut 1-2  in half-ish and make a couple of ~15 round mags for things like standards stages where I have to reload, and then take the bottoms and tack them onto other 30's to make 40+ rounders for long stages.  The mags are pretty thick by the standards of magazines, as far as I can tell.  I am not even sure I will be able to source springs to make the long ones work, but I would like to try.

I welded two Colt type AR15 9mm mags together recently. ASC brand mags, holds 47 rounds and runs flawless. I milled out the top of the follower for the top spring to mount in and then trimmed the bottom spring until I got full compression with the most rounds. I used a 32 and a 25, I just purchased two 32 rounders to do it to next.

I used a MIG welder spotting it on its lowest setting. Jigging the mags up is the most important thing.

A couple of weldless options:

http://www.makereadyproshop.com/clt9mm/

Those are for wimpy little 9mm pansy mags, I need to connect 2 Manly Beefy .45ACP mags...

As a guy with a tig welder, the welding would be the easy part. Cutting, prepping, refinishing, and getting the springs to run 100% would be the hard part. Once you get it ready to weld, the welding is easy.

I wouldn’t want to do an aluminum mag though. Trying to deal with the oxide layer on thin aluminum sucks. If you haven’t ever tig welded aluminum...  The oxide layer melts at a significantly higher temperature than the metal itself. If you have a thick oxide and don’t remove it the metal will melt, the oxide will contain it, then the oxide melts/breaks/ruptures and all the metal underneath flows out like water. With thicker/not really that thick aluminum it is easier to grind away the oxide, wire brush, and weld. I personally don’t like less than 1/8” or really thick aluminum. Thick sucks too as it is a huge heatsink.

Well that's actually great news because there's not a speck of aluminum anywhere near these, they are 100% heavy duty thick American Steel ! I am a little concerned about the springs but I figured worst case I could probably take the springs out of the two donor magazines and make some sort of an intermediate follower that captures the ends of both Plan B is these mags are almost big enough that I could use a shotgun tube spring if I had to, LOL maybe a 20 gauge