I have ds1 (3.8) and ds2 (18 mths).
What I find tricky is the constant vigilance needed about ds1's toys in relation to ds2. Toys do get broken and I have to keep a constant beady eye for eg a little tyre that has come off a car belonging to ds1 that ds2 might swallow etc. It's just not realistic to keep all of ds1's "age 3 and over" toys tucked away, as ds1 wants to play with them.
We always try to buy good quality sturdy stuff, which is less likely to break. Sometimes ds1 has been bought things I confess I've binned because I know they will fall apart in 5 mins, and I don't want to pass them on to another child (via a charity shop say) for the same reason.
I just wanted to check that I'm not alone with this problem which can't really be resolved unless you have separate playrooms for each of them!