Backport PR #54914 on branch 2.1.x (REGR: concat raising for 2 different ea dtypes)

doc/source/whatsnew/v2.1.1.rst

@@ -13,6 +13,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- Fixed regression in :func:`concat` when :class:`DataFrame` 's have two different extension dtypes (:issue:`54848`)
 - Fixed regression in :func:`merge` when merging over a PyArrow string index (:issue:`54894`)
 - Fixed regression in :func:`read_csv` when ``usecols`` is given and ``dtypes`` is a dict for ``engine="python"`` (:issue:`54868`)
 - Fixed regression in :func:`read_csv` when ``delim_whitespace`` is True (:issue:`54918`, :issue:`54931`)

pandas/core/internals/concat.py

@@ -177,7 +177,7 @@ def concatenate_managers(
                 values = np.concatenate(vals, axis=1)  # type: ignore[arg-type]
             elif is_1d_only_ea_dtype(blk.dtype):
                 # TODO(EA2D): special-casing not needed with 2D EAs
-                values = concat_compat(vals, axis=1, ea_compat_axis=True)
+                values = concat_compat(vals, axis=0, ea_compat_axis=True)
                 values = ensure_block_shape(values, ndim=2)
             else:
                 values = concat_compat(vals, axis=1)

pandas/tests/reshape/concat/test_concat.py

@@ -858,3 +858,12 @@ def test_concat_multiindex_with_category():
     )
     expected = expected.set_index(["c1", "c2"])
     tm.assert_frame_equal(result, expected)
+
+
+def test_concat_ea_upcast():
+    # GH#54848
+    df1 = DataFrame(["a"], dtype="string")
+    df2 = DataFrame([1], dtype="Int64")
+    result = concat([df1, df2])
+    expected = DataFrame(["a", 1], index=[0, 0])
+    tm.assert_frame_equal(result, expected)