I do it by stealth...
Get one absorbed in one thing (tv, blocks, drawing), then quietly grab the other one and start an activity down the end of the house, but usually I give them little housework jobs to do with me.
Generally, the older one likes to get stuck into something interesting, but the younger one does all she can to thwart him, knock things over etc. So I say sympathetically - 'here, I'll get her out of your way darling" and he is thrilled to be left alone..
I only get about 30 mins each time, but I break up the day as much as I can. Sometimes I even pit the little buggers against each other to see who can pick up more toys at cleanup time
Although now
DS is more patient I'm looking for activities that will have them working together to achieve things so they can appreciate each other more.