The two quotations do not contradict each other!
I give you concisely theological-ontological reasons:
Father does not give anything without mediation of the Son; thus, the Son's Incarnation, His atoning Sacrifice through His Death on the Cross, His Resurrection - these are absolutely indispensable for salvation, so that Father can save no man without this man participating in Christ's suffering and death, for only thus he will be able to reach Resurrection and Life (cf. Romans 6:8).
This being the case, then, of course, it is both the Father and the Son conferring the enthronement to the conquerers, and those conquerers are made such only through co-action with the graceful presence in them of Christ (1 John 4:4). Thus, yes, one can with absolute assuredness affirm that the Son, Jesus Christ confers - alongside with the Father - the place in the Kingdom to each and every man who becomes a blessed conquerer.
This being the case, then how can we understand the logion of the Savior that "to sit at my right hand or at my left is not mine to grant, but it is for those for whom it has been prepared” (Mark 10:39-40)?
A temptation is to fall to a heresy and affirm that there is some greater authority than the Lord who prepares to humans that placing closer or further from the Enthroned Lord. Who can be this authority? Father? But we have seen that it is impossible, because Father prepares and grants everything coupled with the Son, so their activity with this regard is one divine activity. Then, if not Father, who, Holy Spirit? - no, absurd. Then what prepares? A Necessity? But then we get not Christianity but paganism, for it is in Hellenic paganism that the Necessity is the highest principle, but not in Christianity. Thus also this is exluded.
Here's what: the "preparation" and sitting closer or further does not depend on God any more, neither on Father, nor on the Son, nor on the Holy Spirit, but on human agent and his freedom! Even God does not interfere with human freedom and your closeness to God depends on your free initiative of more intensive co-action and reciprocation with God. For instance, St Anthony the Hermit had a pious sister, and when he heard in Church from Gospel "sell everything and follow Me", being rich, he gave dowry to his sister, gave her to a noble marriage, but himself becoming complete pauper went to caves and spent all his life in ascetic fights and intensive prayers, acquiring gifts of clairvoyance and miracle-working. His sister also finished as a good and pious Christian, but surely St. Anthony became closer to Christ, more saturated by divine grace than his also very good and pious sister and this hierarchical difference depended not on God any more, but on their different free responses to His graceful gifts, for some makes his or her talent more multiplied, some less, and both St. Anthony and his sister are saved, but St.Anthony's free response was more intensive and more selfless, that's why he acquired a greater closeness, and that's why humanity remembers his name, whereas his good and saved sister's name is forgotten (not for God to be sure!). Thus, "preparation" depends not on God but on our freedom, on our free response to God, and this is frightening.
A mystery of freedom is frightening.