HelloCustomerThank you for coming to JustAnswer with your question, and welcome.
At this point, it would be best to let nature take its course. To produce breast milk while supporting a pregnancy, you will need to pay special attention to your intake of nutrients so that you can feed both children without getting run down yourself. You will also need more rest. Your milk supply may dwindle gradually because of the hormones produced during the pregnancy, but you can supplement your child's feedings.
Goat's milk is a good alternative to cow's milk or soy products. The molecules in goats milk are much smaller and more easily digested, much like breast milk. So, you won't have the gas, bloating, constipation and colic that you might get from giving a cow's milk based formula.
The only precaution, and I'm sure you know this, is to get a form of cow's milk that has been pastuerized. If you get it pastuerized in its normal form, you can give it to her as is. If you use canned goats milk, you can dilute it with water without needing to add anything else to it. Then use a half and half dilution. I've used goats milk for 3 of my 5 children when they were babies with very good results.
Your daughter at 8 months old should be taking other foods as well, so this will help with weaning her. You can continue to breast feed her as long as she will take it. If your supply dwindles too much, or the taste of the breast milk changes (it can happen with the pregnancy) she may naturally want to wean herself from the breast.
You don't need to dilute the goats milk unless you get it in a can (condensed). But remember, she will take milk out of a bottle much faster than your breast, so she may accidentally take too much until she gets used to it. Try offering 4 ounces at a time. If she wants more, wait a few minutes and offer more. At her age, she should be able to take 6 to 8 ounces at a time, but she just needs to slow down. You can also burp her frequently during the feeding---babies will ingest much more air from a bottle than the breast and this will cause her to "throw up" with a big air bubble.
I can't advise you on the use of herbs during pregnancy, as these products are not regulated, and have not been studied as far as safety during pregnancy (or any other time). You never know how much is actually in the supplement or tea, as well as there may be other ingredients not listed on the label. In general, my opinion is to always to stay away from herbal remedies and medications as much as possible during a pregnancy.
But, you can ask your doctor about the herbal supplements---if your doctor approves, then go for it.
Here is a link with some information about using herbal remedies during pregnancy:
Herb and drug safety chart It gives you list of herbs to avoid, and a list of herbs to be cautious with (which I would avoid also).
Oh, on average your breasts will produce 2 to 4 ounces each for a child of your daughter's age.