Yes you can use the Amazon cc at Costco, and you will get 1% cashback there with it regardless of Prime (Costco is classified as an 'other/misc' category and not a grocery store, hence only the 1%).
With the Tangerine cc you only get 0.5% at Costco, so your Amazon cc will be better in this case.
Tangerine does have a much larger list of 2% cb categories though than Amazon. I would take a look at this thread:
Ultimate Tangerine MC Category Reference
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