Hi Izzy,
You've got the right idea....solids only need to be incorporated into the babies' diet when their need for calories out matches a mom's greatest milk making capacity...
Whether a mom has reached her greatest milk-making capacity depends on how intensively she is breastfeeding...
Nursing just 8 times a day is the cusp between full lactation and weaning. A mom nursing just 7 times a day is definitely in weaning mode.
So moms nursing just 8 times a day can increase their milk-making if they want too just by nursing more, and that may mean their babies don't need any solids yet.
Moms who are already nursing ten or more times a day are already very close too, if not at , their upper limit of milk-making so if their babies are still hungry after nursing they can be offered solids if they are over six months of age. (Under six months babies would need formula not solids if their mom's milk making ability couldn't keep up with demand)
It is normal for many babies to start going one long stretch at night during months 2-4 because they can get all the milk they need during the rest of the day, but it is also normal for babies to pick back up on their night nursing around the middle of the first year for two reasons,
1) they are bigger and just need more milk, and
2) the milk is higher in fat at night and that extra fat helps myelinate the nerves that are responsible for the big developmental changes like learning to sit and crawl...
Growth spurts are another of those things that are ubiquitous myth- babies' growth rate does nothing but slow down the first year...
There are periods in an older child's life where they grow more rapidly than others, but they don't last just a few days or weeks like we think of when we think of "growth spurts" that seem to happen to babies while we are breastfeeding them...
All the good breastfeeding literature refers to "frequency days" because the increase in breastfeeding during these times has more to do with a mother's milk supply than a baby's growth rate, and these books want to be accurate without discounting the mother's perception of what is going on...
As babies get older we moms feel like doing more and sometimes we get so busy doing others things that we nurse less often or for shorter periods of time so that our milk supply goes down...we don't perceive that this is what we are doing but the babies do...
The babies have to have "growth spurts" where they make us sit down and nurse them intensively for a few days so that we will recover our full milk supply.
So do either of your girls need solids? How can you tell?
One of the girls is exhibiting that she isn't coordinated enough for solids yet so if 8 nursings a day isn't enough for her you have the capapcity to make more milk for her simply by adding in a nursing or two.
With your other daughter if you are nursing her 8- 10 times a day and she still seems hungry you can offer solids AFTER nursing her.
"Nurse first for the first year" is a practice that helps ensure that babies get all the nutrient dense breastmilk they need with minimal dsiplacement by solid food.
So if a mom is nursing more than 8 times a day and nursing first before offering solids she is doing what she can to make sure that she has a plentiful milk supply, and that solids are being offered in additiion to breastmilk and not displacing breastmilk too much from the baby's diet.
If with frequent nursing a baby still wants solids than it is a sign her caloric need has finally outstripped her moms milk-making capacity, but most mothers find that with good breastfeeding management babies under a year old consume some but not a lot of solids.
Mothers of twins might encounter this a little sooner or to a little greater to degree than moms of singeltons but not by much because while a mom of twins does have two babies she usually has a lot more active glandular tissue than the mom of one because she had more glandular development during pregnancy and more stimulation after the babies were born.
After a year of nursing it is okay to let solids slowly displace breastmilk. The World Health Organization recommends that babies one to two years of age get 4-5 breastfeeds a day and 3-4 meals of solid food. If a mom conceives again then the toddler would need to eat more solid food because the milk supply dwindles during the first trimester and there is only colostrum there by the second trimester.
Sorry this was so long but I hate to just make a recommendation to a mom without giving her all the reasons behind it.