i've used 2 sims on my p910, so while not exactly the same i will tell you what was involved, then it might apply
1. i had to cut the sims to size, with scissors.. which was a daunting task in itself
2. i put both sims into the 'holder'
3. booted phone, and it loads a little program asking you which sim to use
4. after that, all was fine
but, i discovered that by knocking the phone a little bit, sometimes the sim got unrecognised, resulting in having to open it up, reseat the sim holder, and boot again and retry until the sim was detected.. overall it was a pain in the ass, and its considerably easier to just replace the sims when you need them
1. i had to cut the sims to size, with scissors.. which was a daunting task in itself
2. i put both sims into the 'holder'
3. booted phone, and it loads a little program asking you which sim to use
4. after that, all was fine
but, i discovered that by knocking the phone a little bit, sometimes the sim got unrecognised, resulting in having to open it up, reseat the sim holder, and boot again and retry until the sim was detected.. overall it was a pain in the ass, and its considerably easier to just replace the sims when you need them