Whether you merge or differentiate cannibalizing pages depends on intent: merge when both pages serve the same intent, and differentiate when they target genuinely distinct intents that simply got blurred. The pivot is same-intent-merge versus distinct-intent-differentiate. So the first move after confirming cannibalization is not to pick a fix, it is to ask whether the two pages are really trying to satisfy the same search need or two different ones that drifted into overlapping territory.
When both pages answer the same intent, merging is the right call because two pages competing for one need just split signals that one stronger page could consolidate. Combine the best of both into a single authoritative page, redirect the weaker URL to the survivor with a 301, and let all the links and relevance flow to one destination instead of being divided. You end up with one page that can win the query cleanly rather than two that keep undercutting each other for the same slot.
When the pages actually serve distinct intents that got tangled, the fix is differentiation, not merging. Maybe one page should answer a how-to question and the other a comparison or a product decision, but their titles, headings, and angles blurred until both chased the same query. Here you re-aim each page to its own intent, rewriting titles, introductions, and focus so they clearly target different needs and stop overlapping. Defaulting to merge in this case would destroy a page that deserved to exist and could rank well once it is pointed at its own audience.
Check intent before you act. If the two pages serve the same need, merge into the stronger one and redirect the other; if they serve distinct needs that got blurred, re-angle each toward its own intent so they no longer compete. Let the intent question, not a reflex toward consolidation, decide which fix you apply.