I think this so highlights where our Western culture is out of whack. I wish there was a team of aunties and grannies round looking after your toddler so you could be with baby. Your baby has no idea why you are leaving him but you've been placed in the unenviable position of having to chose between the needs of two beings who both need you! I feel for you *hugs*. Do you have anyone who could help by preparing some meals for your toddler in advance that you could just heat up, or making a sandwich plate. In fact if someone was able to do that, they could do it for you too!! While it maybe what has to happen sometimes, I am not keen on us all accepting this is normal and OK. We need to start asking others for help and support (those of us lucky enough to have some) in the early weeks and demanding that our partners who share equal responsibility in deciding to have this child also share equal responsibility in taking care of that child - and in the early weeks that means taking care of you, taking on extra responsibility with older kids and doing more domestic work. Sorry I know that doesn't help you much, rant over!
I completely agree with you - if only we all had more help!! My OH will be back at work after two weeks and my mum will only be over a few times a week, so much of the time it will just be me and my 3 kids when the new baby comes. Since we no longer live in multigenerational houses, this is really our lot, unless youre rich enough to afford a nanny.
That said - for the benefit of the OP, it really will not do a baby any harm if they cry for a few minutes while you attend to your older children. It really wont. I say this as someone who has practiced attachment parenting and cosleeping, clearly I dont believe in letting babies cry, but when you have more than one it is an inevitability that at some point one or more of your children will have to wait (which sometimes means crying), while you attend to the needs of the other children. I think we need to make this point very strongly as the other thing we dont need in our western society is any more reasons for mums to feel any guilt over their parenting.
I also firmly believe that it is part and parcel of having a sibling - you dont always get to come first. Sometimes you have to wait and take your turn and I honestly believe this is character building as you grow up. Its hard when its a newborn, but its the situation mums of more than one find themselves in and its not going to adversely affect the newborn if youve already taken care of all of their needs. Older siblings needs are a legitimate reason for putting down your baby - even if they cry. You are after all, still Mum to your older kids and they still need you too, just as much as they did before the new baby was born. You just have to get used to managing needs when you have more than one!
Sorry if I come in strongly about this, but like I said in my first post, the guilt of having to do this for a minute or two at a time absolutely contributed to my PND last time around and Im absolutely determined that other women know its okay and your kids will be fine!