Haiti and the Dominican have remarkably different political histories - Haiti has a lot more awful dictators in its past. They're poor comparisons. Toronto and Detroit are probably even worse - Detroit is an industrial centre whose industry rotted out around the same time as it was levelled(metaphorically) by race riots and people fleeing from them. Toronto is a financial and cultural centre whose primary challenge is that they're short a freeway or two and the commute times are bad. If you want to compare Toronto to anything, NYC or maybe Chicago are the obvious candidates.

As for a close cross-border comparison, I propose Seattle and Vancouver. They're close, similarly sized, and culturally close. You can also look at smaller examples(Sault Ste Marie ON/MI, Niagara Falls ON/NY, etc.), but it's harder to get good data from those due to scale, even though they are even closer in geographic and cultural terms.

North and South Korea just after the split is probably closest to what your looking for, though currently, they have 50 years of differing political histories separating them. Still probably better than most examples.

Both countries share a cultural inheritance from the native Taino people, but how much of that was able to escape post-contact ethnocide is a matter of debate. The two countires were united under first Spanish and then French colonial rule, but following the end of colonialism, the mainly Spanish-speaking east engaged in a hard fought war of independance from the French-speaking west. The "awful straight" border you noticed was the end result of this war.

In my limited-ish experience, some Canadian border towns (Niagra Falls, in particular) get pretty close, but you'll get lots of people on both sides that concentrate on their national identity.