How can two terms be one inside the other without fusing or separating?

Standing answer: Merleau-Ponty does not solve this; he dissolves it. The fuse-or-separate dilemma has force only if one concedes that there are two demarcated terms first, which must then be either joined or held apart. MP denies the premise: mutual implication is not a result reached by operating on pre-given terms but the starting structure prior to which there is no "two" awaiting unification. The problem-site shifts from epistemology ("how do we cross the gap between subject and object?") to ontology ("what is the mode of being of the always-already-mutual-implication that any subject-object framing presupposes?"). The full treatment is the problem-space page mutual-implication-as-ontological-starting-structure; this page is the filed Q&A.

The short answer

Dissolve, don't solve. The two horns are the two failures of "proximal thought" — intuitive coincidence (fusion) and reflective overview (survey-from-above). Both posit two terms and then ask how to fuse or survey them. The *Ineinander* ("one inside the other") refuses both at once by holding the terms already one-inside-the-other before any such operation.
The écart is what keeps it out of both ditches — because the non-coincidence is the bond. Against fusion: the reversibility of the terms is "always imminent and never realized in fact… the coincidence eclipses at the moment of realization" (V&I Ch 4, p. 147). Against separation: the hiatus "is not an ontological void… it is spanned by the total being of my body, and by that of the world; it is the zero of pressure between two solids that makes them adhere to one another" (Ch 4, p. 148). A zero-pressure seam is what makes two solids adhere.
The precise navigational formula (Carbone, Chiasmi International 4 [2002]: 57, via chiral-reversibility): "It would be as bad to think of a reversibility without écart, that is available to be realized as peaceful confusion of the related elements, as it would be to think of écart as a fracture… [setting] their absolute distinction and therefore their reciprocal extrusion." Peaceful confusion = the fusion-pole; fracture = the Cartesian-distinction pole; the écart steers between them.
Why "ontologically primary": Nature Course 3 (p. 239) — "Natural being is a hollow [creux], because it is the being of totality." A totality cannot be one of its own parts, so it shows itself as absence-from-within. Dews fixes the modality: the écart is "a hollow and not a hole" — relative, binding non-coincidence, not absolute lack.

The two ditches, refused by name

Against Cartesian distinction → *empiètement*. MP's one-line self-definition of philosophy, from the note left red-underlined on his desk the night he died: "L'empiétement, qui est pour moi la philosophie, n'est pour Descartes que confusion, c'est-à-dire néant" (DESC 84). What pensée distincte throws out as "confusion, that is to say nothingness" is exactly what MP makes philosophy be. Per Saint Aubert, empiètement is the genus of chiasm / Ineinander / reversibility.
Against undifferentiated identity → chiral reversibility. Right-hand-glove / left-hand-glove: "ontological complicity without identity; the gap is necessary to the structure." This blocks the Hegelian "self-restoring sameness" reading — the reciprocity is "an open series of reflections," not a circle closing on itself.

Your five pairs are one form in five registers

Pair	Register & anchor
self / world	ineinander: "inherence of the self-in-the-world or of the world-in-the-self, what Husserl calls the Ineinander" (In Praise of Philosophy p. 182).
visible / invisible	chiasm: the invisible is "the relief, the depth of the visible" — within, not behind.
animal / human	Nature C3 p. 225: emergence "in the Ineinander with the animal… by escape and not by superposition" — not hierarchy-by-addition. See interanimality.
past / present	Nov 1960 working note: "past and present are Ineinander, each enveloping-enveloped — and that itself is the flesh" (claims#nov-1960-stiftung-grammatical-subject supported).
philosopher / philosophy	The fold / invagination: the question is "truly of the world that it questions, fold in this fabric." Philosophy is "the knowledge of the Ineinander."

That the same structure does all five jobs without category mistake is the evidence it is ontologically primary, not regional (claims#ineinander-universalizes-institution supported).

Where it is genuinely contested

Morin's "desire for the One" (claims#morin-ecart-as-encroachment-vs-unpassable-limit live): at V&I 148 MP may undercut his own écart by reasserting flesh as a grounding principle behind the divergence — i.e. he may not fully escape the fusion-pole.
Nancy: a properly thought écart is the unpassable limit "without depth or thickness"; MP's "logic of promiscuity" risks evacuating genuine separation.
Saint Aubert's surrection: the Ineinander gives the topology of flesh but not its dynamics; absent surrection it risks a "molluscan ontology of indefinite circulation without genuine birth or separation."

The structure that answers the question is central and well-attested; whether MP's execution tips back toward identity (Morin) or needs a separation-restoring supplement (Saint Aubert) is exactly where current scholarship lives. The wiki holds these positions side by side without adjudicating at V&I 148.

What Remains Open

The Morin V&I 148 contestation is unresolved by design; a targeted re-read against the French Gallimard text (the wiki has only the Lingis English) is the open philological task (General Rule 18).
How far the structure extends (plants, non-living bodies, the cosmos) is undetermined — the interanimality / flesh-as-element reach-question.