diff --git a/lib/matplotlib/_constrained_layout.py b/lib/matplotlib/_constrained_layout.py
index 90faebc1525a..2b99010b64c6 100644
--- a/lib/matplotlib/_constrained_layout.py
+++ b/lib/matplotlib/_constrained_layout.py
@@ -403,9 +403,25 @@ def _align_spines(fig, gs):
                 if height0 > height1:
                     ax0._poslayoutbox.constrain_height_min(
                         ax1._poslayoutbox.height * height0 / height1)
+                    # the precise constraint must be weak because
+                    # we may have one col with two axes, and the other
+                    # column with just one, and we want the top and
+                    # bottom of the short axes to align with the large one,
+                    # so the height of the long one is greater than
+                    # twice the height of the small ones. OTOH, if the
+                    # axes are not so constrained, we would like the pos
+                    # boxes to be the proper relative widths.
+                    # See test_constrained_layout17 fo
+                    ax0._poslayoutbox.constrain_height(
+                        ax1._poslayoutbox.height * height0 / height1,
+                        strength='weak')
                 elif height0 < height1:
                     ax1._poslayoutbox.constrain_height_min(
                         ax0._poslayoutbox.height * height1 / height0)
+                    # see above
+                    ax1._poslayoutbox.constrain_height(
+                        ax0._poslayoutbox.height * height1 / height0,
+                        strength='weak')
             # For widths, do it if the subplots share a row.
             if not alignwidth and len(colspan0) == len(colspan1):
                 ax0._poslayoutbox.constrain_width(
@@ -415,9 +431,17 @@ def _align_spines(fig, gs):
                 if width0 > width1:
                     ax0._poslayoutbox.constrain_width_min(
                         ax1._poslayoutbox.width * width0 / width1)
+                    # see comment above
+                    ax0._poslayoutbox.constrain_width(
+                        ax1._poslayoutbox.width * width0 / width1,
+                        strength='weak')
                 elif width0 < width1:
                     ax1._poslayoutbox.constrain_width_min(
                         ax0._poslayoutbox.width * width1 / width0)
+                    # see comment above
+                    ax1._poslayoutbox.constrain_width_min(
+                        ax0._poslayoutbox.width * width1 / width0,
+                        strength='weak')
 
 
 def _arrange_subplotspecs(gs, hspace=0, wspace=0):
diff --git a/lib/matplotlib/tests/test_constrainedlayout.py b/lib/matplotlib/tests/test_constrainedlayout.py
index 46e6b9663ef3..95fb4e8779c5 100644
--- a/lib/matplotlib/tests/test_constrainedlayout.py
+++ b/lib/matplotlib/tests/test_constrainedlayout.py
@@ -399,3 +399,20 @@ def test_hidden_axes():
 
     np.testing.assert_allclose(
         extents1, [0.045552, 0.548288, 0.47319, 0.982638], rtol=1e-5)
+
+
+def test_two_under_one():
+    fig = plt.figure(constrained_layout=True)
+    gs = fig.add_gridspec(2, 2)
+    ax0 = fig.add_subplot(gs[0, :])
+    ax10 = fig.add_subplot(gs[1, 0])
+    ax11 = fig.add_subplot(gs[1, 1])
+    fig.canvas.draw()
+    extents0 = np.copy(ax0.get_position().extents)
+    extents10 = np.copy(ax10.get_position().extents)
+    extents11 = np.copy(ax11.get_position().extents)
+
+    np.testing.assert_allclose(extents0[0], extents10[0])
+    np.testing.assert_allclose(extents0[2], extents11[2])
+    np.testing.assert_allclose(extents11[0] - extents10[2],
+                               0.077362, atol=1e-6)