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Make two shapes with linear blends, remove the stroke, and join them together. For your example, one is red and white and one is white and blue. Align them next to each other with the white portions touching. Play with the sliders. Group them. Enclose them with a no-fill shape if you want a border.

This is easy for rectangles and perhaps other shapes with at least one straight side. For more complex shapes you may have to cut one in half (e.g., using subtract shapes), then duplicate, flip, and join.