I've a sub panel in my shop and I ran a new conduit down one wall in preparation for a saw station. There are two circuits in the conduit, that tie to two separate GFI breakers. I have independent power (black) and neutral (white) 12/2 strand, with a common ground (green) -- 5 wires were fished through. There are two boxes each with two duplex 20 amp receptacles. One circuit is on the left receptacle duplex, and the other on the right in both boxes. If I bypass the first box and only wire the receptacles in the second box (furthest from the panel), it works fine. If I wire in the first box, any load on any of the receptacles in the first box, it will trip both GFIs (a tester shows the receptacles are wired correctly). If I leave the first box wired in, and put a load on any of the four receptacles in the second box they all work fine. Any ideas?

Post is not clear.
In the title you say GFI breakers trip.
Are you using GFI breakers and regular outlets or are you using regular breaker and GFI outlets? If you are using GFI outs, how many, two or four?

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any load on any of the receptacles in the first box, it will trip both GFIs

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Does that occur when nothing is plugged into the outlets of the second box?

If you are using GFI breakers does that mean the breakers trip or does it mean the GFI portion of the breakers trip.

Check you neutrals. If you have hot 1 and neutral 1 to an outlet in the first box and hot 2 and neutral 2 to the other outlet in box 1, but have hot 1 and neutral 2 to an outlet in the second box and hot 2 and neutral 1 to the other outlet in the second box, That would amount to a shared neutral. GFIs don't like shared neutrals.

Two GFI breakers and regular 20A outlets. When the breakers trip there's no indication as to whether it is the GFI that tripped. They just trip. I'm pretty sure I'm not sharing neutrals between the two circuits, but I'll recheck. Maybe I did swap hots and neutrals like you suggested.

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I'm pretty sure I'm not sharing neutrals between the two circuits

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Let us know. If both breakers are tripping, can't think of any other logical reason.

Using GFI outlets would have been cheaper.

It ended up being swapped neutrals. I swapped the neutrals in the first box, and the problem moved to the second. I swapped the neutrals in the second and presto; it all works. The one thing I noticed is that my outlet/GFI tester won't detect swapped neutrals (maybe none do?). Thank you for the prompt responses.

Congrats.

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It ended up being swapped neutrals

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When you do that the little electrons get confused and can't find their way home.