That sounds like pretty normal newborn behaviour to me. When he's having one ounce, sleeping for half an hour, taking another ounce, sleeping 20 mins, having another ounce then sleeping again, I wouldn't class that as taking a 3oz feed (not sure if that's what you meant though), I'd say that's three 1oz feeds. When he's having 3oz over an hour without sleeping, I'd think it was really the same thing - he is probably taking it so slowly to space it out to the same effect as doing 1oz followed by a short sleep. Newborns do often want feeding small amounts really often. It's a pain to do it but it's really best just to go with it if you can possibly do so, because his tummy is probably only big enough for an ounce or two at a time. That 20 min sleep after an ounce (or taking it really slowly) allows his body to process the milk and then he'll wake for more. If you get him to take 3oz at once and then sleep for longer, it may be more than his tummy can actually cope with. In a week or two you'll probably find he will take more at once/quicker and then sleep a bit longer, you just have to find a way to get through it for now
If you're finding it difficult to look after your older LO at the same time as endless feeding, can you put together some little activities you can do together while feeding your youngest? Some sticker books maybe, some jigsaws, some pictures cut from magazines and a glue stick?