What about the two thieves crucified with Christ?

Very little is known about these two men. The are mentioned in the gospels, but only Luke records any interaction they had with Christ at the time of their death. Commonly, one is called the repentant thief and one is called the unrepentant thief. This is because although they both reviled him (as recorded in other gospels), one thinks better of that and realizes that both of the thieves are being executed for their crimes and that Jesus has done nothing worthy of death. He receives grace and mercy from Jesus who declares that that thief would be with Him in paradise that day. It is always assumed that the other thief who reviled Christ went to hell.

The only fact that we know is that at least the one thief went to paradise with Christ because Jesus said it. When the darkness fell over the land and Christ died, offering up His Spirit to the Father in heaven, it was enough to change the hearts of even a few Roman guards and at least make them consider that Christ was the Son of God. Whether they did anything with this is not recorded. But keep in mind that the same events, coupled with the testimony and faith of the "repentant" thief, could have been enough to change the heart of the "unrepentant" thief as well, and that he could have accepted Christ as His Savior before he died. It is known that Christ died first, because the legs of both other thieves were broken. So we really have no knowledge of whether Christ saved one or both at His death. The only thing that we know for sure is that He at least saved one.

Never assume that you know the heart of another. Only God knows the true condition of each human heart and whether or not that person has accepted Christ as the means to bridge the gap between God and the sinful state of mankind.