tr = IndirectTransform(
    lambda: Bbox.union([
        txt1.get_window_extent(),
        txt2.get_window_extent()
    ])
)
This allows any artist to be positioned relative to the combined bounding box of multiple other artists - which is exactly what #22223 requests.

##What changed
Added IndirectTransform class in lib/matplotlib/transforms.py

## Testing

import matplotlib.transforms as T
bbox = T.Bbox([[0, 0], [1, 1]])
tr = T.IndirectTransform(lambda: bbox)
print(tr._resolve())  # BboxTransformTo(Bbox(...))

## AI Assistance
1. Understand the existing transform hierarchy in 'transforms.py'
2. Trace through the 6 cases in '_get_xy_transform()'
3. Draft and refine the initial implementation