Well, when they share a room your older ds can keep his stuff on higher shelves or in the closet or dresser tops. I have 4 yos and a 18 month old. I allow the boys to take their legos out only when the baby is down for a nap or I'm in the room...same with markers, paints, glue, etc.
My daughter is 21 months old and she seems to be out of the "everything goes in the mouth" phase.
My son is 6 and he has alot of toys with small parts. We made sure he understood that he could not leave these laying around because his sister might put them in her mouth and choke. After a couple reminders, he became really conscious of where he put his toys and even started to make sure there was "no bad stuff his sister could get" when we laid her down to play.
I think by the age of two, your baby will be fine in a room with his older brother. By this age, they aren't trying to eat everything
I agree with lil' love. I have a 2yo and 6yo, and they pretty much know the limits on what can be left around and what is hands-off. If you have a playpen or one of those fence-type things that makes corrall in the middle of the room (eesh, what a vague description) you can have the "dangerous" stuff in there, and let the baby explore the rest of the room.
Our biggest problem is with friends of the 6yo who don't know the house rules and don't have younger siblings. They sometimes leave taboo stuff laying around or leave doors open that should be latched shut.
At 26mos. the little one is less interested in putting small objects in her mouth than in upsetting her sister by grabbing beloved toys and running off with them.