Hi there, (((hugs))) that does sound hard
No magic bullet here, I'm afraid, but I have a few thoughts/comments/suggestions to start with...
Do you think he may be in any sort of discomfort? Reflux, for example? That can certainly make it difficult for LOs to settle. Or gassiness? Constipation? Rashes/allergies? Teething?
What steps have you been able to make towards independent sleep? If he's in discomfort, if for example lying flat causes him pain, then we'd always advise to get that sorted first. How does he currently fall asleep, when he actually does? It will certainly be easier in the long run to work towards independent sleep, but obviously difficult with a toddler around. Would he sleep in a sling/pram at all? At this age, you could perhaps aim for one nap where you sleep train whilst your toddler watches TV, and then another nap where you APOP in sling/pram so that your toddler can get outside. BT will be difficult of course, I have no btdt with that age gap but I know many others here have, so they're bound to have useful suggestions
But please don't feel that you're failing your boys, I know it's so hard but it will get easier, this phase will pass, I know it feels like forever now but one day it will just seem like a blur and you'll start to enjoy watching them playing with each other, making each other laugh, and you'll know it's all worth it
At the same time, know that you're not alone in feeling that way - many of us have and I've personally come here in tears many times and found wonderful support, I hope you will find the same