The Court

The court of Francis died with him. Anne de Pisseleu having been banished, kept busy trying to hang on to her property she had neither time nor influence to bother with Jarnac. Also the old court members were scattered to their estates to lick their wounds and remember the old days,

The new court was probably in turmoil. The various offices had to be given out and fought over. All the furnishings need to be rearranged scrapped and bought. Stuff would be lost and other stuff found.  On top of all this scurrying around they had to prepare for the coronation in the Cathedral at Rheims at the end of July.  Dianne came into her own. She started gathering all the spare change in the kingdom and passing out offices to her friends.
The Court's preparations for the duel would be almost as huge as the coronation, since it was to be it a showpiece of the upcoming reign. The place chosen was the Royal ChЙteau of Saint-Germain-En Laye, which stood on a hill overlooking Paris.

Chataigneraie

Chataigneraie was part of the new court so he had many duties and lots of networking to do.  Everyone knew that he would win, so no special preparations on his part were necessary, even if he knew what Jarnac was planning. But Jarnac was away from court, out in the family country estates and obviously no visitors were allowed. The armor/weapon list was loaded with false information, so would be of little help, but since he had to buy it, whether it was used or not, would cost a lot.
Some authors have implied that Chataigneraie did not practice at all. But since he was in the circle of a king who loved to hunt, joust and play tennis, this was unlikely. But we do know that he held Jarnac's skill in contempt and felt that he had less to fear “... than a lion fears a dog. ” However, he kept one hundred twenty five of his buddies around him at all times
Chataigneraie decided to celebrate the victory after the fight with a sumptuous banquet to which the court, attending dignitaries and his friends would be invited. He had a large pavilion erected beside the lists complete with tables, chairs and auxiliaries including gold serving dishes.   He not only used his own facilities but borrowed stuff from his friends and also, it was reported, from the King.

Jarnac

Jarnac had ended up in a rather neat position. Originally, he was the challenger so Chataigneraie would have had the choice of weapons. However that had been lost in the paperwork so that now he had the choice of weapons. In addition he was no longer needed or wanted at court. He needed to develop fighting skills and a weapon/armor system that would give him a chance of surviving. This required time and money. He retired to his father's estate and set about the preparations.
First thing he did was to develop a deep sense of piety. So even if the Lord did not help him survive, at least he would be prepared for finals. Then, convinced that God helps those who help themselves, he hired an Italian Master of Defense, Captain Cazio. Then they set to work. As De Bazancourt says,  all they could to do in such a short time was perfect a simple system of defense and a couple of attacks - but these had to be perfect.  So the training was probably limited but intensive.