I don't. Mine are about the same age, 5.5/2.5 right now, and we have a system worked out.
They may play in the sand box, which I can see from the garage. They may play on the trampoline, which I can see from the garage. They may play in one very specific section of the drive way, right in front of the garage. They must play together, and may not move out of the places I can see them. My kids are also quite loud usually, and so I can hear if they move out of the designated areas, or if they stop fighting I know the younger one ran off, but the big one usually tells me pretty quick. Yes, it kind of makes him responsible for his sister, but he is very responsible in general, especially for his age, and he has really kicked it up a notch in the last few months here because he really wants another sibling to torture. I think he is hoping he can train the next one to be on his side because his little sister can already beat him up.
If the big one is good about staying right there and working with you, but you need to keep the little one occupied let them draw on the garage floor with some chalk.