You'll be able to figure it out. Your mortgage advisor is not very creative it sounds like. Here are a couple suggestions:
1. If your next purchase is a true multi-family (duplex with 2 meters), then you will indeed need to put 20% down even if you will owner-occupy one side. However, if you purchase a regular single-family home with a mother-in-law unit in the basement, you will be able to use a 5% down conventional loan. The key is what the property is officially zoned as.
2. Regarding the problem of having too much debt to income due to your existing mortgage, here is one possible way to get around that. Many banks will consider the rental property a wash if you can show them a signed rental lease to prove that you will have income on it--even if those renters have not yet moved in. What you need to do is find a renter to sign a lease for your house in advance, say for $1600/month. The bank will look at that signed lease and count 75% of that rent income against the mortgage. On their analysis, it will bring in $1200 against your $1300 mortgage, thus leaving you with only $100 of monthly debt. If you can do this, it will free up your debt-to-income ratio to qualify for the next purchase.
I just saw Chris' post, exactly the same idea. Hope that helps!