Imho the one on the left is superior as long as you have the ball valves before each reactor. As mentioned above the two medias need different flows through them. I have the brs dual chamber reactors and I think I'm going to swap it out for 2 seperate reactors so I can adjust for optimal flow though each media. I like your idea of one pump split to the 2 reactors though.

If you want an even cleaner setup bleed some water off of your return pump to feed the reactors individually.

if you want proper flow to each media with only one pump then a manifold is the only way to do it......carbon and gfo have different optimal flow rates and "daisy chaining" them together doesnt work.......I've always used two separate pumps

Yeah I'm going to do the manifold in my new build for sure. But I also have 2 Reactors hooked up to my FOWLR, so I think i'm going to do a manifold for that one too. I'll just have to do some quick re-plumbing.

That being said, does GFO have to tumble?? And faster or slower flow through the carbon reactor? I use the Kent Reef Carbon, so it's bigger pellets. Right now it's got enough flow through it to lift it about 3/4" off the bottom of the reactor.

flow to carbon should be around 400-600 GPH and flow to GFO should be around 200-400 GPH....personally Ive only ever used "Rowaphos" and its supposed to look like boiling water when it has proper flow, so definitely not aggressively tumbling....cant speak for other kinds/brands of GFO

Oh wow, thats a lot more flow than I thought. Looks like i'm due for a plumbing upgrade! I have a Maxi-Jet 1200 running two TLF reactors lol.

i have mine off a manifold from return pump works like a charm

i had issues with clumping using option 2 i vote or option 1