I was wondering i can use multiple frequent flyer programs for different legs in the same itinerary. any comment/advice would be appreciated. thanks!

If you want a short answer it is easy:
NO!

Why do you want to know this? There may be strategies that don't require a mid trip program change (e.g., if you want lounge access from 1 account with status but want the credit to go to another non status account)

im taking AC WAS-YYZ-HKG but I cant accure mileage (A fare) with UA from WAS-YYZ so I was wondering if I can use another program, like NH or LH for the WAS-YYZ segment and use UA for the rest to maximize mileage accural.

Just go to a transfer counter before the flight you want to have the alternate FFP on and ask them to change it and print you a new BP.

One word of warning though: My experience is that it's a 50/50 chance they will change not only the leg you ask them to change, but the rest of your itin, so you better go to another transfer desk after the flight and make sure it is changed back to whatever you want to use afterwards.

Not that easy and not correct. You can do this as agehall points out, but as s/he says be aware that the more complicated you make it, the more likely there will be an issue.

What about not putting any FF numbers during booking/checkin and later claiming missing FF credit for each segment separately? It seems like a less risky strategy than switching FF numbers with agents.

Retro claiming is an option as well, but it can be a pain and sometimes you really have to fight to get the miles. I'd avoid it because of the hassle, but it is a somewhat safer option.

just completed my trip and it turned out that most CSR, especially those in the lounges, were very nice about changing your FF# but as some of you mentioned, you will have to do it for each leg.

I am *G on AP, trying to migrate to A3 (and maybe subsequently to MP). I am currently on the second leg of an itinerary with US. I am connecting through PHX. I have tried to get my FFP changed at each of: customer service desk, gate, lounge, and exited to check-in counter and ALL CSRs refused to change the FFP for second leg (at gate, both CSRs emphatically agreed that what I was requesting was 'impossible'). Interestingly, each had slightly different reasons why not, but I was unable to find anyone who was willing to even attempt to implement the change.

Is there anything I could do differently to get the miles credited to another FFP?

There are some other threads where I may cross-post this for input.

Flew recently with AC LAX-YYZ-CPH. Had our TK FFP *G in the system. At LAX we'd want to credit the flights with UA (better mileage - and we are UA S). At LAX agent couldn't do anything, she said the best bet was to call AC's CS. At 8pm Pacific Time I had to queue more than 2 hours to get through to AC's CS. I gave up.

Arrived at YYZ, I found a staff who told me that it should have been possible and she told me to queue at B18/21 (can't remember) at the check-in area. Finally was my turn and I explained to the staff that I really need the YYZ-CPH to be credited into UA. He first said it's impossible since the journey has started. He was very suspicious why a *G want to credit his/her miles into *S account - I explained to him that I have already re-qualified which is true. Then he went on saying that there are frauds going on with people trying to get 2 accounts credited with one leg.

I asked what if he removes it, and he said, 'the previous leg is already history since the journey has completed - so the miles will be credited to TK'. I said fine since I won't get much with UA anyway.

He removed all the FFP, then I said, 'Since there is no FFP, can't you add them in?' He said I'll try and he got our UA S printed on our boarding passes!