I don't know what size bed you have or how you all sleep, but when my dd#2 was born last year when dd#1 was 3, we all still co-slept. I just put baby between my husband and myself and dd1 on the other side of me. We still sleep that way. Sometimes, if I have to get up with dd#2 during the night (she's 15 mo now), when we go back to bed, #1 has rolled over by daddy. When we get back in bed, that's the only time the two girls sleep next to each other, and they usually drape legs/arms over each other and I don't even worry about it anymore because dd#2 is big enough to not be bothered by big sister.
It can be done, and if you can sleep between your two kiddos, it is still pretty safe. If you're worried about your older boy falling out of the bed, you can always put up one of those safety rails on your side, or push your bed up against the wall. I know both seem extreme, but I'm sure it'd only be for a short time.