Separate out what is for freeplay and what is for grouptime when you are there to help such as games. Freeplay here means you choose from whatever I have put out (down on the playtable etc.) for you to use. The rule is you play and I am free to play doing what I want too and it isn't playing with them. Then when our freeplay is done we have time to play together and that is when puzzles, board games, things with small parts are brought down and they are done in a controlled environment on the table, on a mat, tray, etc. I am right there to supervise, help and insist they be put away before moving to the next activity. These things are usually used while babies are napping.

You might also want to eliminate any toys when the older one is there that involves or leads to a lot of confusion/movement as that may be setting him off. Again this would be a good time to have them doing their own game on their own tray in their own space and making sure there are large areas of open space by moving larger items away when the whole group is there.