Just behind the two cabinets. You will then need a clear if some sort to extend behind the fridge level with the top of the cabinets to support the counter.

Hallett probably means cleat, not clear.

IKEA sells counter supports for fridge locations.

Also if you hang using the rail, be aware that it pushes your cabinet out 1/2" (I think). With the cabinet box at 24" and full overlay drawer faces/ cabinet doors at 1/4", we had almost no overhang. Had to do some quick thinking.

Our counters were not purchased from IKEA so that might be why. We bought 25" wide butcher block

If you haven't already, I suggest using the IKEA design tool and having the IKEA designers check it for you. There were a few things we wouldn't have known to get (like the counter support, and the cover panels for the fridge). Here's ours.

For 2 lower cabinets on a base, with a space in between them, I am not sure I'd even bother with the rail and just use the direct mount method. Build the base, level the boxes and screw them into the wall. (assuming you have something solid to screw into...) . If it's standard sheetrock and studs use the rail.

However- if the cabs are not very wide you COULD have an issue with the rail not having enough solid attachment points and that might necessitate using the rail full width to get enough screws in studs.

Yes, we had to take down Sheetrock and add blocking to do ours. Didn't have enough studs since our wall was just furred out.

Thank you , everyone! It's a standard drywall and studs wall, with unfinished, empty space on the other side to make sure attachments actually DO go into the studs. :)