I have twins, too, and I hear your frustration. It is tempting to push this stuff along and try and get more breaks, less breast feeding, more sleep. The whole thing is exhausting! But, if you just started yesterday, then they should only have had one feeding of 1-2 tbs (per child), generally in the morning. You continue that schedule for at least 2 weeks, giving their bodies the opportunity to gradually adjust to how to process the new food. There are two things that are going on in this first stage: 1) they have to learn how to push the food from the front of their mouth to the back of their mouth and then swallow without choking, and 2) their digestive system has to learn how to process proteins and other nutrients other than what is found in breast milk. They are not deriving actual nutritional benefit from the rice cereal (or any other solid food) at this time. Thus, if you give them too much, you will be essentially replacing a food source (breast milk) with a non-food-source filler (rice cereal), and you are depriving your daughters of essential nutrients. So go easy on the quantity of food at this early stage.
At 5 months, your girls also lack the hand-eye-mouth coordination, not to mention the swallowing skill, to self-feed. If you think it's messy to feed them rice cereal, go ahead, offer them a bowl of pureed pears and see what happens. Eventually, they will develop the coordination to try and feed themselves, but that's months off for you, and once it happens, things get very very messy indeed as they will insist on trying to feed themselves and the food will go everywhere.
You should talk with your pediatrician about this, but the general rule for starting solids is rice cereal once a day (usually the morning) for two weeks, by the end of which time you should be increasing the feeds to 2 tbs of cereal and reduce the milk so that it looks more like oatmeal and less like thick milk. After two weeks of that, add a second feed, also 2 tbs, in the evening. It is at that point that you [may] start to see some improvement in their sleep as a result of the evening feed. (There are mixed messages as far as whether or not to put cereal in a bottle, but generally it's not a good idea b/c it's a choking hazard.) When you add the second meal, you can also introduce a fruit puree; a lot of people start with pears or sweet potatoes, either of which is fine. Stick with one food, added to the cereal & breast milk, for at least 3-4 days and preferably a week to allow their systems to adjust. After that, you can add a new food (fruit or veggies only until at least 9 mos) every 3-4 days, watching closely for any signs of allergy (rash on the face or elsewhere on the body, wheezing, hives... your pediatrician should have given you a list of things to look for). You can also switch to oatmeal (baby oatmeal, not adult oatmeal), which is thicker and tends to keep them satiated longer, and is also often less constipating than rice cereal. Over the course of the next few weeks increase the feedings to about 4tbs of cereal plus fruit/veggie puree. After a couple months of this, you can add a third feeding at lunch time. For us, the third feeding was always a veggie w/out the cereal/breast milk added to it, to change up the flavor and texture a bit and increase their experimentation.
So, yes, eventually the solids that you introduce will reduce the amount of breast milk that they consume, but that takes time and you need to go slowly in order to ensure that your girls are getting what they need. Right now you're training them and their bodies to process foods. And please don't give them cheerios as someone suggested at this stage: there are way too many ingredients for them to process, not the least of which is refined sugar, and plus they're a choking hazard for several more months. You will know your girls are ready for finger foods like cheerios when they have had wheat and other grains (after rice cereal and oatmeal you can introduce a multi-grain cereal, but remember not to introduce more than one new food every 3-4 days), and they have the pincer grasp skill to get the cheerios into their own mouths.
Also, as your child's system develops, you can start introducing new foods more quickly, like every other day. That usually starts around 9 months.
Your pediatrician should also have given you a list of foods to avoid for the first year or so. Usually on that list are: strawberries, refined sugar, tree nuts and nut butters, cow's milk (you can introduce whole milk yogurt at about 9 or 10 months and pasteurized cheese at about 10 or 11 months), tuna and honey until at least a year; shellfish, peanuts and peanut butter until at least 18 mos. I hope I'm not forgetting any, this is just off the top of my head. I'm sure others (and your pediatrician) can add to that list.
Sorry this is so long, I hope it's helpful. Good luck!