Been there, 3 times. Here's what you do.
During the day make sure your toddler gets some bursts of exercise like doing 10 jumping jacks or running the length of the backyard and say something like "wow you are using lots of energy, your body is going to need lots of sleep tonight!" and leave it at that, don't open the door for arguments.
Start getting ready for bedtime 20 minutes early - Add A. extra special big brother routine to bedtime where he helps you sing night to the baby and then you and the baby sing goodnight to him.
Then - Put the infant in a wrap on your chest, pay no attention to the baby while you are going through your normal bedtime routine with your toddler. Your infant may go to sleep on your chest or will at least be content listening to your heartbeat. Wrap the sling tight enough that you can still pick up and bend down to tuck your boy in bed. It takes some time learning to maneuver with a baby sling but it's worth it and you won't feel like you are needing to pay attention to both children.