It's very simple, in my opinion.
He told them to go and bring both animals. Matthew, however, only tells us that He requested the colt. His omission of the the other animal does not make what he records wrong; only incomplete.
In Matt 9:18KJV ff Matthew condenses the episode about Jairus' daughter and then again about the woman with the issue of blood.
Luke gives a lot more detail Luke 8:41KJV ff but we do not suggest there is an error simply because Matthew gives less information.
I am reminded of John 21:25KJV. Undoubtedly both Luke and Mark would have left out some details as well.
But your question is a different one. Your question is: Which one of the two things Jesus is recorded as saying, did He, in fact, say?
Could He have said;
"Loose them both and bring them to me. Bring the colt which has never been ridden that I may enter Jerusalem to fulfill prophecy and bring the other one to lead it."
In this case both Matthew and Luke would have recorded correctly different aspects of the same command.
I appreciate your post and understand what you are saying.
For those who believe God breathed the words, these (at best) incomplete statements (in the case of the animals) or contradictory statements (the centurion came and spoke or the centurion did not come and speak) should be a billboard announcing their beliefs are inconsistent with scripture.
I have not brought up the events with the daughter of Jarius as yet. It is one of the few, insignificant errors that I also point out when trying to wean people from the false doctrine of inerrancy.
Your suggestion of what could have been said just leaves all the gospels misquoting Jesus.
And the error in Matthew misinterpreting Zech 9.9 also persists.
I think the credibility of the gospels is enhanced by these minor errors. They show a lack of collusion and revising.