I've done both. When I used to run in college (85 to 120 miles per week) I ran twice per day 5 or 6 days per week, usually 3 to 7 in the morning and 7 to 12 in the afternoon, with races on saturday and long runs on sunday. These days, I'll double a couple of times per week sometimes, but most doubles have one workout as cross training. Most runs are in the 5-8 mile range, with long runs on the weekends. I'll take a day off every week or 3, weather dependant.

I find going out for 5-8km easy on 'rest' days helps more for injury prevention than doing nothing, myself.

Same for me. I would add too that I believe those slower "rest" miles improve overall fitness, and if weight is an issue it all they help you to stay thin. No need to shorten longer runs either, imo, to make room for a few extra easy miles.