Hello,

They should come on all together  (the two in front of the condenser, and the one in front of the intercooler, with the addition of the auxiliary electric pump when the engine is off) in my experience, at around 97-98 °C (before 100 anyway) when aircon is off. With aircon on, they should always be on.
I myself have never noticed two speeds on my BMW-equipped Omega.

I did post before on this, now I cant remember the exact sequence but its something like....

Aircon on eco no fans will run until car reaches 92 Deg
Aircon on, at least one fan wil turn over at low speed all the time
At 92 Deg A fan (or maybe 2 I cant recall) will come on at half speed until the temp goes down to something like 85 (or thereabouts)
at 95 Deg one or more fans comes on at full speed. You will know this is happening as its a bit noisy, kind of last resort cooling.

In my case it was getting to 95 then the high speed kicked in, never got a low speed at all. Turned out to be a blown fuse in the engine bay near the battery.

ETA: I found the thread, check this out it has a picture showing which fuse to check http://www.omegaowners.com/forum/YaBB.pl?num=1221845157/8#8

hi again
looked at bigdodds reply and the link posted, shot outside to the old girl and what do you know the fuse he pionted out had gone. changed the fuse took her out for a good run and the temp now stays around the half way mark (95-97). the fans then stayed on for a short time after i had stopped so all in all im over the moon that this has cost me 40p to sort out. bigdoods and theboy, you are legends, this forum is the best thing ive ever seen on tinternet and anyone who has a mig should come here long before booking their car in anywhere!
many thanks