So, our situation is kind of different...we are a foster-only program, and most of our kittens come from local shelters, where they are already exposed to the same diseases and may have already been matched up with non-siblings.
However, almost never do we stick a singleton in foster care, ESPECIALLY if there are no other cats in the house. The only exception would I guess be a FeLV+ kitten. We have had one in the history of the program and he was in a quarantine room in a house with other healthy cats and kittens, and he was also older, about 6 months I think.
In my opinion, being a singleton kitten (we call them only lonelies) is one of the worst things for a young kitten. It's not only bad for their social well-being but I think that in turn affects health and development as well. We do our best to test non-siblings for FeLV before grouping together but the chances of a positive in our area is very low. We try not to mix directly-owner surrendered with shelter-obtained kittens because the shelter kittens are all exposed to URI already, as well as god knows what else, and the owner surrenders may have a chance of not getting URI. However, they can go to the same foster home, we just don't house them together.
But if the shelter calls and says hey, we have a 4 week old who needs you, and they have nothing else in the remote age range, we will take it, but pair it as soon as possible with another kitten as one becomes available. It is sooooo much better for them, even if they get each other sick. I would honestly rather have a kitten give a buddy URI, or giardia, or coccidia, then be solo. I can treat them both together and they'll bond all the more for it. The only time I stop is when I suspect panleukopenia, because I'm not giving another kitten a likely death sentence just for buddyship. However I've only ever had panleuk in groups to begin with, anyway.
Takeaway--Pair and Trio and Quartet those babies! It's good socialization, it's good to learn how to be a kitten, especially when a mom isn't around to model that behavior. FeLV test before-hand if possible but if not, don't fret unless it is common in your area. Kittens don't have to be the same age but within a few weeks is good if they are neonates. Track weights always to make sure older kittens aren't hogging the food if they are free-feeding. Treat URI and runny poo issues generously but they are rarely the end of the line for a kitten in foster care. Kittens over 8 weeks can be paired easily too. Once they get past 3-4 months if they have been solo their entire lives, it will be more difficult but can (and IMO should) be done. But with slower introduction. Around 4-12 weeks we usually just toss them in a q cage together (we use large multi-level ferret or cat cages for quarantine) with plenty of resources. Yes, that means multiple litter boxes and multiple food and watering stations, and multiple beds. No resource guarder can successfully protect everything from the others and they learn quickly at that age it's unnecessary.
Lucky for us, our main partner shelter feels similarly to us and frequently pairs and trios the kittens before they even reach us. (It is also a space saver for them, of course).
So I would go ahead and send two singles to a foster home with no quarantine/separation period. Especially if they come from the same source. Heck, send three or four. No point in limiting it to only two. If you get singleton bottle babies, that sometimes happens, but make sure they have cuddle buddies--a stuffed animal their size (we have beanie baby-type stuffies up through larger kitten sizes for this express purpose), but if you have two only lonely bottle babies, why not stick them together? If a foster is getting up every 2, 3 hours to feed it doesn't take that much extra time to feed two versus one. And don't bother trying to test the bottle babies for FeLV...too hard and stressful, IMO, to try to hit a vein and I don't believe in other methods of gathering blood for testing. When they are that little, they need every drop. Test when 4+ weeks of age when they're more robust and can spare it.
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