Several factors are considered in loans as significant as a home mortgage. I believe the most major factors are 1) Credit report, 2) Income, and 3) Employment status
If you borrow jointly, all joint factors are included, not just the favorable ones. Some wrinkles this can cause may include:
Credit Report - The second person on the loan may have poor credit or no credit. This can/will hurt your rate or even prevent them from being listed on the loan at all, which will also mean you can't include their income.
In addition, there are future consequences: that any late payments, default, foreclosure, etc. will be listed on all borrower's reports.
- Employment - When I purchased a home, there were rules about the borrower being employed in the same industry for a certain amount of time. I qualified but my wife didn't.
If you both have solid work history, great credit, and want to jointly own the home, then there shouldn't be any negatives. If this is not the case, compare both cases (fully, not just rates, as some agents could sneakily say you can get the same rate either way but then not tell you closing costs in one scenario are higher), and pick the one that is best overall.
This is just information from my recollection so make sure to verify and ask plenty of questions, don't go forward on assumptions.