The mortgage interest deduction limit is for the mortgage debt. In your example you share a total mortgage debt of $1,500,000; the limit on that debt is $750,000. If you split the debt 50/50 that's a debt of 750,000 each and so the limit is split 50/50 as well.
The bottom line result is the limit 750K/1.5M = 50% of interest is deductible on your total mortgage debt. If you each paid 50% of the interest, you each can deduct 50% of your interest paid.
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