Separate because weeds die best when they are healthy. When you fertilize with weed n feed, the fertilizer kicks in more slowly but the herbicide tries to work right away. By the time the weeds are healthy enough to die, the herbicide is depleted. The other problem is that 90% of the people don't follow the directions on weed n feed.

It is better to fertilize about now and wait two weeks to spot spray with a liquid herbicide.

Why do you ask?

Pre-emergent or post emergent?
Granular or have you found some liquid wonder?

Singles because:

1 - Sometimes you only want to kill a few weeds, not fertilize the lawn. Or the lawn is weed-free but needs fertilizer.

2 - Getting the right weed-killing dose may mean over or under-fertilizing the plants you want to keep, or at the wrong time for their growth cycle.

3 - It's cheaper to buy single-purpose products.

+1 lazygardens