Impact of Double Workouts...

I do doubles several times a week. Don't really track the impact of each session on the other and I don't really notice much unless one is a particularly hard session though I try to break them up, and almost never do hard session back to back.
So, I'd say don't fret it too much.

I do doubles a couple times a week as well and find little impact if I'm mixing swimming with running/biking.  If I run twice in one day, the second session seems to start slower and I'll likely end up adding 15-30 seconds to my pace.  If I'm doing a double on a day with a long/hard workout, I do it first and make the second workout lighter.  Generally I listen to my body and slow the pace down on the second workout as needed.  I really don't see a lot of difference between doing a double and an evening - morning workout combination.  They all blur together after a while.

I frequently do two-a-days, though usually with 10-12 hours in between.  Personally, I don't find that a morning or lunch swim workout affects my evening run much at all (unless I haven't been sleeping enough and would just rather go to bed).  However, when I do an intense or long run I don't plan an intense or long bike as the next session as my legs don't feel recovered enough. 

Brian

My double yesterday was bike commute to work in the morning and back home in the late afternoon/early evening (about 22 miles each way).  Energy and fatigue were fine yesterday through both rides and beyond, but I'm pretty worn out today.  Swam this morning and had a decend workout, but definitely wasn't at 100%.  I usually have a few doubles in my week, and they don't really wipe me out, I think yesterday's was on account of some more intense biking volueme than I've been used to.

x2 - really the only time it is difficult is the swims thursday are usually tough (typically for me, it is a long swim, 3000+ yds) because wednesdays are usually an hour + runs in the morning and 1:30-2 hr ride at night.  friday morning are tough too because thursday evening track workouts tear up my legs quite a bit. .

come to think of it, pretty much all of these two/days have been tough while i have been working with a coach (2 months)   i like doing 2/days because i just feel better all day.  as far as effecting performance - i don't know because its what i'm used to doing.

I would say no impact at all.

Swimming is low impact and if it's a few hours before the second workout... no biggie.

It really depends on your swim fitness. If you don't have fitness built up for a 2,500 yd swim and it's a breakthrough workout for you because your longest swim was only 1,000 then I would say yes. However, that usually is not the case with most people.

Additionally, I have not seen any research showing a significant impact on running performance when completed after a swim.

From a practical standpoint, I almost always have my athletes run and bike on separate days (with a few workout or specific trng exceptions) which leaves their swimming always being on the same day as either a run or a swim.