Scrub potatoes and place on a rack in a 5- to 6-quart pan over about 1 inch boiling water. Cover and steam over medium-high heat until potatoes are almost tender when pierced, about 15 minutes.
Rinse squash and trim stem ends. Cut squash in half lengthwise. Set squash on potatoes, cover, and continue to steam until vegetables are tender when pierced, about 5 minutes more. Add boiling water to pan as needed to maintain water level. Let vegetables cool.
Meanwhile, husk, rinse, and thinly slice the tomatillos. Stem, seed, and dice the red bell pepper and the poblano chili.
When the potatoes are cool enough to touch, peel and cut into pieces no thicker than 1/2 inch. Put potatoes in a wide salad bowl. Add orange and lime juices; mix gently, taste, and if desired add about 1 teaspoon sugar (or to taste).
Add tomatillos, red bell pepper, chili, and cilantro to potatoes; mix gently. Cut squash halves in half lengthwise, add to salad, and mix gently again. Season potato salad to taste with salt. Serve while still warm or at room temperature.