Guru is "one", in that there is no difference in the objective of diksha and siksha gurus. Their aim is to assist their dependants in their advancement by destroying ignorance with the torchlight of knowledge. In other words, the diksha guru wants to see his disciple advance towards pure devotional service and the siksha gurus want to see their siksha disciple advance towards pure devotional service. They are happy- especially when siksha comes in Srila Prabhupada's line - even if the disciple receives the strength and knowledge to advance from a source other than himself. Therefore from wherever strength and knowledge come, the disciple should take it - with the blessings of his guru. There may even be many who provide that to us at different stages of our advancement.
Collected Letters to Friends and disciples Vol 4, Pg 129, 1st paragraph.