aggression between two schnauzers

Question
Good Afternoon,  

I have a 15lb., 11 year old miniature schnauzer.  My friend, she has a 28lb. 5 year old miniature schnauzer.  We have tried to bring these two dogs together for about three years without aggression.  They see each other about twice a week for about 5 hours at a time.  Many times they are relatively fine then there will be a fight.  These two dogs don't like each other.  Please offer any advice for we would really like to have these two get along.  The bigger dog also has snapped at me on two occasions lately.  

Answer
Hi, Barry,

Thanks for the question.

What are the genders of the two dogs? Are they intact or have they been surgically altered? What kind of games does your older dog like to play? Is your dog's energy still good, or is s/he slowing down a little with age? How long have these two been seeing each other? Are they ever taken on long walks together, preferably in as natural a setting as possible? What do they fight over: sleeping place, a bone, a biscuit, human attention, or what?

I look forward to hearing back from you.

LCK