We’re thinking of joining forces and hope we can find a house with an accessory dwelling unit as an additional rental income.

I'm not sure how it is in your area but if you have the option to buy a single family home with an accessory/in-law apartment, that could be a good way to start.

Whether single, or those with families, House Hacking is possible with multi-family or with Single Family Homes with an Accessory Dwelling Unit (ADU)It is great that you are trying to adjust and be more intentional about your finances to secure a better for yourself and family. 

clarification to above:  there are lots to accommodate 5-6 separate houses with accessory structures or 2-3 times more apartments/condos.

In 2018, we put an Accessory Dwelling Unit in the basement that has been rented long term since January 2019. 

Depending on where you live, what you may be looking for is a house with an ADU (accessory dwelling unit).

I did that prior to having a family - now we live in a house that has a detached accessory dwelling unit out back so we are still house hacking but staying put for now 

If your neighborhood doesn't demand quartz counter tops with gold accessories...then there's no reason to pay for those items.  

I reached our to a mortgage lender & he is saying the only way I can purchase a multi-family is to purchase a single home with a separate accessory unit on the property (like a separate cottage or such) versus a duplex/triplex.I need to take advantage of the 3.5% down in order to make this work.

Why would it have to be a separate accessory on the property vs a duplex/triplex?