I have taken both of these drugs.
Before i was diagnosed with ME/CFS i was working fully time, living full time and went to my doctor struggling to cope all the demands (in hindsight - not surprising) she prescribed 20mg of Citalopram, which over the next year went up to 40mg.
It did work, to some degree. It made me feel quite emotionally numb. This brings its own problems, but it did help me.
Then after being diagnosed with ME/CFS and having a sleep study, the sleep doctor listed three drugs to try to improve my sleep. All she said, worked well with people with ME.
The first she suggested was Amitryptaline and starting at 10mg and rising to 20mg later. It came with a warning. That was that in the first instance, the first two weeks i would feel awful on it, and that most people discard the drug in this period thinking that it doesnt work for them but makes them worse.
She explained that this was normal, and you should presevere because after two weeks the side effects disappear and then you get the benefit.
She was spot on.
The first week was dreadful, i spent all of it on the sofa. I would have given up had she not prepared me first. The second week was also not good. After that though, the side effects vanished. The only thing that i am left with is a slightly dry mouth, and a fast beating heart (i.e. 100 at rest) and its a little punchy five minutes after taking the tablets at night. This is all minor though, especially when balanced against the benefit.
I now sleep REALLY well. I wish i knew about this drug a decade ago. Maybe then i wouldnt have got ME/CFS as im sure this contributed to wearing me down.
Personally, i cannot recomend the drug enough for sleep. Doseage wise, for depression alot more is given, and high doses are also given to children and its widely used...so i figure that its pretty safe.
Two months ago i decided to come off the citalopram, firstly because i wanted to take less pills, second because of the cost (although on prescription, the additional 7.40 every other month is hard to find right now), thirdly because i read about long term use of SSRIs being nto good for you, and fourthly because i felt ready to deal with coming off it. I did it VERY slowly, and finished coming off it about three weeks ago. I feel no different - no worse - so thats a result.
So both are widely used, and can benefit you, especially the amitryptaline, in my experience. Both can be taken together so long dosage is set properly, but i agree with other posters that its best to start one at a time. They both effectively increase the seratonin in your brain, Citalopram by stopping the seratonin you produce naturally, from being broken down, instead it is taken back up (this probably isnt the best explaination, you might want your doctor to explain it better) and Amitryptaline works in a different way that i dont remember as well.
Personally, I would recomend Amytriptaline for EVERYONE with ME/CFS who has sleep problems. Of course, some people with ME/CFS dont tollerate any drugs well, but if you are going to try any, i would try this for two weeks if you possibly can, as the side effect to benefit ratio is hugely in favour of the benefit in most people who take the drug.
Hope that helps.