I would highly recommend seeing a lactation consultant.  They are expensive but totally worth every penny.  Mine gave a huge discount to twin moms.  My twin girls are also 9 weeks old and I am exclusively breastfeeding on demand.  I had a lot of difficulties in the beginning and the LC was extremely helpful in helping me to up my supply, ensure a good latch, and just supporting me!  At this age, babies should be gaining an average of 4-7 oz per week.  You could try...