Not much advice, but sympathies - both my DCs have done this. You'd really think that babies would be able to sleep as and when they needed it, wouldn't you.
With DD I found that the sling worked, if I kept walking.
With DS (6 mo) I am now using the sling for a morning nap and then feeding him to sleep on my lap for a lunchtime nap. I keep him on my lap and every time he wakes up I pop him back on the boob and he goes back off. He is sleeping up to 2 hours like that. Then I don't bother trying to get any more naps and I feed him to sleep at 6pm, sometimes earlier.
This only works since DD is at pre-school till 3pm so I can afford to spend 2 hours of my day doing nothing! But on the days when DD is home it's really hard, and she watches way too much cbeebies whilst I try to get DS to nap and deal with the wailing that inevitably follows from too-sort naps.
So basically, the only things that have worked for me are:
sling whilst walking, no stopping
constant breastfeeding
early night instead of late afternoon nap.
With 2 DCs at home, obviously the first two options are not workable unless your older DC will come on a walk in the buggy or watch a dvd whilst you feed. To be honest Roseability, I might even consider getting childcare for a few hours a day in your shoes, if you could find a method like breastfeeding that worked.