Triathletes, especially the ones training for long distance events, do doubles almost by necessity. Heck, if anyone who took up triathlon were independently wealthy they'd get up at 8, swim from 9 to 10, then go for an easy one-hour ride or a 30-minute jog, followed by a long, lazy afternoon in front of the tube talking to the dog. HOWEVER...most tri-geeks (and some of their coaches) have real jobs. We would much rather talk about the trip we've just returned from or the next event we're going to do; it's lots more fun than talking about your doughnut-munching co-workers (or cow-workers, as one of my athletes likes to call them...) or the last batch of paperwork you had to turn into smaller batches of paperwork for the benefit of your company.
The challenge comes in balancing the morning effort & the evening effort. In order to make double workouts most effective, so I am learning, is to ensure there is enough time to recover so you're not completely wiped out or on the fast track to an overuse injury. Even if you decide to separate your run workouts with a cross-training effort (for triathletes, the non-impact sports of triathlon, swimming & cycling) it's still important to adjust between hard efforts, easy efforts, long durations & short ones. If doing the same workout at the same intensity over the same duration twice a day doesn't bore you to the point you decide to not get out of bed after the first week...it will certainly beat you down so you end up with...yep...an overuse injury.
Nutrition & hydration are also important, before, during and in between workouts. Extended endurance training may burn off fat, but too much can break down muscle. So, you want to make certain to eat good food throughout the day. Someone like Don Kardong might recommend you avoid a diet that discourages the use of ice cream (okay, at least you can have dessert in moderation); your body ends up chasing down anything it can burn after a certain point in time...and you don't want it to be muscle.Oh, and there will be times when you have to listen to your body to shut down a workout when pain arrives. You'll know pain, it feels much different than discomfort. There'll be days when you absolutely have to rest. Do it. There's a good reason. Sometimes you have to do the one-mile test: Go out for one mile, or perhaps the first half mile out & back on your planned run. If you still feel bad call it a day & rest. If you feel fine after a mile, go right ahead & continue...but take an extra easy workout or two.
As you approach the target event, you might want to cut back to one workout a day during the week or two weeks just before...your body won't know what to do with the extra energy come race morning.