Assuming all 4 joints (both hips and knees) need replacing, would they be done all at once? If not would it be by leg or joint?
Not that I’m any sort of expert, but I’ve known people who had simultaneous knee replacements, but never a simultaneous knee and hip, much less both knees and both hips. The recovery process would be a nightmare!
Lacunae Matata-(what a “wonderful username”) 
So, am I right that doctors would do 1 joint at a time?
Anecdote != data… but a friend who had both knees done last year did have them done at different times. I think they only want you to be dealing with one bum joint at a time. Not to mention you’d be looking at double (or more) surgical time / time under anesthesia on a single day which of course would increase the risks. And with both sides, there’s a greater chance of being totally unable to get around which has its own set of problems.
Not a knee replacement but: my SIL had carpal tunnel surgery on both wrists, some years back. She wanted the doc to do both at once (to minimize lost time from work) but he pointed out “and just who will be wiping your ass for you???”.
She laughed and went with doing 'em one at a time.
As for doing knee+hip on the same leg, I don’t know. I’d guess that would not be their first choice for all the reasons above, but I’d bet they’d be likelier to do that, than to do both knees / both hips.
Like Mama Z points out, anecdote isn’t data, but I’ve known a couple of folks who had both knees replaced at the same time. (One was a 78 year old woman who was in the rehab home at the same time my aunt was recuperating from her knee replacement. Nice lady, and she recovered/went home sooner than my then-58-year-old aunt.) Never heard of anyone having both hips or a knee and a hip done at once.
My husband had his left knee replaced last summer, and in retrospect, he wishes that the surgeon would have agreed to do his right at the same time.
Simultaneous bilateral (both sides at once) hip replacement is quickly finding favor as some studies are showing better outcomes than in staged (one side at a time) procedures. Not everyone is a candidate for a simultaneous procedure - you do have to be “under” for longer, and if you have cardiovascular disease, diabetes or other medical conditions, that might be too dangerous. But if you are a candidate, studies are showing better outcomes in walking with simultaneous replacements, with equal complication rates as procedures staged less than 6 months apart, and lower complication rates than procedures staged more than 6 months apart.
Knees are…less clear. The rehab is much harder for knees than hips due to the type of joint, and without a “good leg” to make rehab easier, knee patients often prefer staged bilateral replacements. Besides the rehab issue, there’s pretty good evidencethat simultaneous bilateral knee replacement is not as safe as staged procedures, leading to greater incidence of pulmonary embolism, cardiovascular incidents and even death. On the other hand, many patients would prefer to “get it over with” in one procedure, and doing it simultaneously means only one hospital stay, one round of anesthesia and one period of time off work - real benefits to real people. It’s definitely a “talk it over with your doctor” situation.
I can’t find any studies on simultaneous knee and hip replacement on one leg or two. I’m sure it’s been done in cases of trauma, but I don’t think it’s done for replacement of arthritic joints. It’s going to carry all the risks of simultaneous bilateral knee replacement, and increased risks of blood loss, fluid imbalance and deep vein thrombosis.
As the Baby Boomers age, it’s expected that rates of hip and knee replacements will rise, so I would not at all be surprised if new techniques are developed. In another 10 years, we’ll know a lot more.
The hospital where I work (which does a ton of hip and knee replacements) will not do two replacements at once.
Notwithstanding WhyNot’s cite above, the orthopods feel that simultaneous replacements pose too many problems during recovery as opposed to unilateral procedures.
Personally, I’d rather do one at a time since you would be, effectively, immobilized for at least a short while if done the other way.
It may change. Who knows? But for right now…one at a time.
I had my left knee replaced in Jan 2010. My doc was willing to do both. but said that getting the left one done might improve the right one enough that I could get another 10 years on it without too much pain. He said that age, (I was 47) and physical condition were a couple of the main determiners for doing both at the same time.