How to split things is the easiest of your problems :)
Ownership is proportioned by contribution to equity, both to the down payment and the mortgage payments. And you both "pay rent" for the parts of the house you each use.
But you're still going to have to pony up a fair bit of cash between you as a down payment (as per this Government of Canada page)
So that's $25k + $40k = $65k.
If you're serious about this, then next thing you need to do is figure out what the income and carrying costs are. Grab amortgage payment spreadsheet and start playing with payment schedules to see what your monthly carrying costs are going to be (not forgetting to set aside cash for property taxes, insurance, maintenance) and then compare that to the potential rental income.
I dare some that many thousands of us here have made a first SFH rental purchases such as this with barely enough money and little idea as to what's involved, so I'm not saying it's a bad idea - just that you need to think hard about the money involved.