But why are Moses and Elijah two thieves, once assumed that there is an implicit rivalry between them and Jesus?
I had thought that Marcion was the author of the episode, to give reason of the his antithesis.
But dr. Detering has a better solution: Moses and Elijah were usurping for themselves the title of “True Prophet” in pre-christian times. Hence, God proclaims that only Jesus/Joshua is the “True Prophet”, not Moses, not Elijah. This solution has the great merit to explain why Joshua is 100% expected in a list where the names of Moses and Elijah appear.
Considering again the my interpretation of the two crucified thieves as Moses and Elijah, then they were "thieves" insofar they were taken themselves as Christs. In the place of the true Christ.
Apart from this, I see a curious clue in the Transfiguration episode in Mark 9:
1 And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power. 2 And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them.
3 And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them. 4 And there appeared unto them Elias with Moses: and they were talking with Jesus. 5 And Peter answered and said to Jesus, Master, it is good for us to be here: and let us make three tabernacles; one for thee, and one for Moses, and one for Elias. 6 For he wist not what to say; for they were sore afraid. 7 And there was a cloud that overshadowed them: and a voice came out of the cloud, saying, This is my beloved Son: hear him. 8 And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves.
3 And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them. 4 And there appeared unto them Elias with Moses: and they were talking with Jesus. 5 And Peter answered and said to Jesus, Master, it is good for us to be here: and let us make three tabernacles; one for thee, and one for Moses, and one for Elias. 6 For he wist not what to say; for they were sore afraid. 7 And there was a cloud that overshadowed them: and a voice came out of the cloud, saying, This is my beloved Son: hear him. 8 And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves.
The fuller is just the final killer of James the Just, according to Hegesippus ("Matthew").
his raiment became shining, exceeding white as snow; so as no fuller on earth can white them.
...may be a cryptical way to allude to a celestial crucifixion:
the criminal action of the archons on Jesus couldn't make him more glorious than he already was.