Is a conventional mortgage an option when I already have one?

[Jeremy Chaser]

Long interest in rental ownership, and I believe I live in a great area for investing in a duplex/triplex given the strong rental market.

My concern is that I already have large mortgage on my primary residence. I make about 80k a year and have a mortgage right now for 234k / $1550 a month. This was about the maximum I was approved for when I applied, but I based my salary off of my base income (65k/year) and disregarded commission. 

I have a high credit score (~790), consistent commission earnings putting me at 80-85k/year, etc. I have about 100k in liquid funds available, approximately 80k in current home equity, 70k in retirement accounts. Overall net worth of about 250k.

My concern is cash flow. I spend about 2k/mo on my home, so that + all other life expenses doesn't really leave much for a second mortgage. The duplexes I'm looking at are in the 200-250k range. I could obviously put 20% down, but how do I go about getting financing? Are there different rules at play if I can show the house has realistic rental potential?

I'm also only late-20's so I don't have much experience or age on my side, which I believe is a negative for me when trying to obtain financing.