不合法, hiding income is a serious thing

Don't do it. It will not be worth it if you knew the punishment for it.
1. I assume you took itemized deductions before. Because you reported mortgage and interest under itemized deduction, you couldn't take standard deduction. However, this time around, you will report rental income, mortgage, interest, repair, depreciation, commission and travel related to the rental in Schedule E. This will let you take the full amount of standard deduction for both you and your spouse. So it won't be too bad for you.
2. Any apartment owner in his/her right mind will not help you do this for nothing. They will only laugh at it.