kaka,
understand your concern, but you can definately increase your milk supply! don't worry.
first of all, how about your pumping schedule? assume you will pump twice at the office, lunch time and after work, then please pump as much as you can, and before, you can drink some soya bean milk and more water.
also, suggest you pump one side and feed another side in the morning or after your work at home, then you can save more for your stock, and also can increase your milk supply( like feeding the twin, and it can tell your brain that your baby needs more milk. ) . or if you can woke up at midnight, also feed one side and pump another side( i know you will be tired also, but it works).
then after one or two weeks practise, you will see the improvement .
now my bb is 13 M, but I insist pumping twice at my office to maintain my milk supply though he drinks less than b4 . so keep it up!