Long stopover search tools

saajan_12

Are there any tools where you can search and buy tickets with a long stopover eg a couple of days? 

Would be a good way of combining two mini holidays in one. I've looked at multi-city options as well as booking two return trips, both of which are more expensive and the two returns option means separate tickets, which can be risky if any delays. I'd think if return, stopover flights are competitively priced for every day in a given week, the same flights a few days later could also be priced similarly. However can't see how to search for or actually book these without going to a travel agent. 

Example: say from London I want to stay for a week in San Fran and 3 days in NY:
1) London - San Fran, short stopover in NY (ie without time in NY) cost £550
2) London - San Fran - NY - London (booked as a multi city, the first leg also stops for a short time in NY) cost £700
3) London - NY return cost £500 plus a separate NY - San Fran return cost £300 so total £800

However I'm looking for option 1, but the return stopover being 3 days instead of 3 hours..