I think that one parallel between Voltaire's Candide and Orwell's 1984 is the intellectual sojourn of each protagonist. Both are presented with philosophies that undergo questioning over the course of the work. Candide questions the optimism with which he is presented through Pangloss's teachings. This philosophy does not explain much in way of human suffering, or why Cunégonde endures what she does, and is not something that he is able to rely upon at the end of the narrative. The initial philosophy with which he is armed is insufficient to address that which he faces. In 1984, Winston is constructed in a similar manner. His initial philosophy of wishing to undercut and undermine Big Brother and the Party is what animates him in the first phase of the novel. However, when he is captured, tortured, and forced to recant everything and everyone he once believed, he is left at the end as an almost gutted shell of what he once was. The final scene of him sitting alone at The Chestnut Tree, vaguely recollecting a memory of his past and completely devoid of anything related to his initial philosophical premise represents how he is physically and intellectually alone. Candide and Winston both experience a philosophical emptiness at the end of each narrative.
A significant aspect of this emptiness is seen in the issue of love. Both works take rather harsh approaches towards love. In Voltaire's construction, Candide's love of Cunégonde is the reason he is banished. He pledges his loyalty to her several times and vows to return to rescue her, and does marry her at the end of the narrative. Yet, the love that they once shared is vastly transformed. Voltaire does not provide a fully restorative and coherent vision of love at the end of the narrative. The love that was once experienced and something that both would have pledged their lives towards has been fundamentally altered through experience, betrayal, and abandonment. This same experience is seen in the love that Julia and Winston share in 1984. Both experience rather intense and dizzying heights of passion, only to find that when they both are tortured, they have both betrayed one another. Their reconciliation for a moment at the end of the narrative is not a reflection of the love they once shared, but rather of the emptiness they both live. When both of them say that they betrayed "the other" and that the care for another is notably absent afterwards, it is a reflection of what once was will never be.
It is interesting to note that in both works, physical displays of affection in the form of sexual exploration is frowned upon by the ruling authorities. Candide is expelled because of his sexual attraction and actions towards Cunégonde, while the O'Brien and the secret police arrest Winston and Julia while they are making love. In both narratives, an interesting parallel emerges in how the prevailing authority sees sex as an act of rebellion, something that is going to disrupt the Status Quo.
Another parallel that is evident in both works is how the mask is torn off of the notion of war. Voltaire does not succumb to a romanticizing of war. Rather, Candide shows it to be barbaric and cruel, reflective in statement such as "the ground was strewn with brains and severed arms and legs." War is an exercise of power, something that Candide himself recognizes throughout his experiences. There is no glorification of the heroism in war, an exercise that is brutality personified. In much the same way, Orwell shows that war is the health of the state, something that Oceania continues to wage only to ensure that the Party and Big Brother remain in power. War is shown as a means to continue and perpetuate the Status Quo, for no one is allowed to raise objections or dissent in a time of war. Both works parallel one another in how they question the structure that causes war and speak to how this condition must be scrutinized.