While Thailand is quite loose, lenient, and lacking in enforcement, days spent is not a big focus in Canada to determine tax residence. You can be Canadian tax resident simply by having residence available in Canada or other vague connections. Days spent is more of a last resort in Canada, for people who don't tick any other boxes but still spend 183 days in Canada.
The Canada—Thailand tax treaty defines residence as:
1. For the purposes of this Convention, the term "resident of a Contracting State" means any person who, under the laws of that State, is liable to tax therein by reason of his domicile, residence, place of management, place of incorporation or any other criterion of a similar nature.
In your case, you are probably tax resident in both according to this clause.
2. Where by reason of the provisions of paragraph 1 an individual is a resident of both Contracting States, then his status shall be determined as follows:
- a) he shall be deemed to be a resident of the State in which he has a permanent home available to him; if he has a permanent home available to him in both States, he shall be deemed to be a resident of the State with which his personal and economic relations are closer (centre of vital interests);
- b) if the State in which he has his centre of vital interests cannot be determined, or if he has not a permanent home available to him in either State, he shall be deemed to be a resident of the State in which he has an habitual abode;
- c) if he has an habitual abode in both States or in neither of them, he shall be deemed to be a resident of the State of which he is a national;
- d) if he is a national of both States or of neither of them, the competent authorities of the Contracting States shall settle the question by mutual agreement.
Just follow those clauses to figure out where you are tax resident. It wouldn't hurt to have a tax lawyer help you figure out where you would fall in your case.
Based spending 5.5 months per year in Canada, I can see Canada insisting you are tax resident there under a, b, and c, making it up to Thailand to fight for you to be tax resident there instead of Canada.
The term permanent home isn't defined in the treaty. But if you manage to spend 5.5 months in Canada, wherever you've been living might qualify as permanent. And if you're spending nearly equal amounts of time in both countries, it might be hard to convince Canada you have your centre of vital interests in Thailand (why don't you live there all the time if it's your centre?). This takes us to clause 2(c) and habitual abode. We're probably back to 5.5 months being long enough time in Canada that wherever you were staying before counts as a habitual abode.
In the end, you're left with clause 2(d) to save you.