2:1 FF into 1:1 MF

100 mm focal length lens for both setups.

Setup 1: 26 MP 2:1 magnification on full frame

Setup 2: 102 MP 1:1 on medium format

How much do I have to crop with setup 2 to achieve setup 1's 2:1 magnification?

Magnification actually doesn't change with sensor size. Just how much of the thing you see.
The term "Magnification" has a strict definition and doesn't change. So 1:1 will show that object at true representation on the sensor.
Your example is interesting in that the pixel pitch is the same for a 26Mpixel APS-C sensor and a 100 Mpixels GFX camera. So you will have the same magnification, but just see more of the object moving to larger sensor and will be at same detail.

Now if you want the images to look the same, just crop the gfx image to the same number of pixels as your APS-C camera.
Example XT-3 is 6240x4160 pixels.
Gfx100 is 11648x8736 pixels.
So the height is croped by 2.1x and the width by 1.87x.

sorry just saw your 135 format sensor is 26 MPIXELS, don't know why I read that as APS-C.

So you will have more detail with the gfx image, both will be same magnification. Gfx100 is equivalent to a 61 Mpixels 135 format camera.

HAND.

As mentioned above, magnification doesn't change with sensor size; only how much of the object is visible. Cropping, sensor resolution, and sensor size have no effect on the magnification ratio of object size to image size. Magnification, in this instance, represents a ratio of the actual size of an object compared to its apparent size in the image plane. So, a ratio of 1:1 is always 1:1, regardless of any other factors mentioned as part of the two setups in the scenario.

Because magnification only represents the ratio between two measurements (object size to image size) it is dimensionless in and of itself. Magnification will change with modification of the lens focal length, lens to object distance, or lens distance from focal plane.

Thank you for a good mathematical explanation.

As others have stated, which I sort of knew, is that magnification is the same regardless of sensor size but I meant something else and you read my mind.