Clearly the first step for Pompey and Ceasar staying allied is Julia not dying in 54. Pompey adored Julia. Pompey also appreciated how Julia's ancient bloodline brought credibility to his provincial Picentum background. Ceasar also adored his daughter Julia. So both powers have incentive to play nice.
Julia birthing a son who lives also goes a long way to helping too. As a previous poster pointed out that such a child would be Ceasar's heir. This gives Ceasar incentive to play nice with Pompey. Pompey already had 2 or 3 sons, so while still an incentive for him, probably not as much as it does for Ceasar.
I don't think anyone has mentioned the importance of Crassus staying alive. Its still a triumvirate at this point as he didn't die till 53. Crassus is regarded as the richest man in Rome.
If Crassus survives his campaign against the Parthians to return to Rome, no matter Pompey's personal dislike of Crassus and jealousy of Ceasar's victories in Gaul, I think his calculation of where power is in Rome would keep him in the triumvirate and he doesn't jump ship to the Optimates.
Now if Crassus not only survives but returns to Rome with solid victories against the Parthians, Pompey's calculations swing hard to staying with the Triumvirate. I'd imagine large successes in the East would make Crassus even outshine Ceasar's wins against those measly barbarians in Gaul.
A Pompey that stays aligned with Ceasar will be extremely jealous of Ceasar's success, and even more so of Crassus if Crassus survives. He will want a campaign of his own.
If Crassus dies, would Pompey go East to avenge Crassus (even though he hated him)? Crassus was Ceasar's friend and historically Ceasar was about to campaign against them when he was assasinated IOTL. Its close, but I think Ceasar would let Pompey go against the Parthians. It gets Pompey out of Rome and surely Ceasar would come back to run Rome as Consul while Pompey is gone. Ceasar would likely calculate the odds as pretty good that Pompey could also die out in the East, leaving Ceasar as undisputed First Man of Rome. Then he gets his campaign of revenge against the Parthians.
If Crassus doesn't die, Pompey will even more demand a campaign to restablish his bona fides as THE war maker of Rome. Would Ceasar and Crassus allow Pompey to manufacture and lead a war against Egypt? Egypt is the BIG prize to Rome. Not yet conquered by Rome because no significant Roman wants any other Roman to get the credit and wealth for having conquered it. Are there any other places Pompey could make war against that might satisfy his ego? Maybe Dacia is the only possibility I can think of? Or if Crassus didn't fully defeat the Parthians, then he goes there (much like he took over from Lucullus against Mithridates)