Dooku was also apprenticed under Dath Sidious, but does that mean he never had any Jedi masters? No. Every bit of Star Wars literature, outside of the only two inwhich Thame Cerulian appears, points to Dooku being Yoda's Padawan at some point; especially Yoda: Dark Rendezvous
. Not to mention Episode II where Yoda sais, "Fought well you have, my old Padawan.
" Yoda does not consider every Youngling he has ever trained to be Padawans, because they are not. A padawan is a one-on-one apprentice to a Master, and Yoda would never have addressed Dooku in this way unless he had - in fact - be his own padawan at one time or another.
Also, it is not uncommon for a Jedi to have many masters throughout their career as a Jedi Padawan. Revan comes to mind as being the greatest example of this. However, the facts tend to point in the direction that Dooku had been the Padawan learner of both, during different times, before he became a Jedi Knight.
Also, this might help...