In the Holy of Holies, only one high priest could enter, is that correct? Sorry, but then why did Nadab and Abihu they entered together (Lev. 10,1-2)? An explanation for this contradiction?
A number of points:
First of all, the Torah does not explicitly say that Nadav and Avihu entered the Holy of Holies. See what it does it say (Vayikra 10:1-2):
"Now Aaron’s sons Nadab and Abihu each took his fire pan, put fire in it, and laid incense on it; and they offered before The LORD alien fire, which had not been enjoined upon them. And fire came forth from The LORD and consumed them; thus they died at the instance of The LORD."
And later their bodies are said to have been removed from the Holy, not the Holy of Holies (ibid. 4):
"Moses called Mishael and Elzaphan, sons of Uzziel the uncle of Aaron, and said to them, “Come forward and carry your kinsmen away from the front of the sanctuary to a place outside the camp.”"
The sanctuary here is how the term קדש (Kodesh/Holy) was translated.
Although I haven't gone through all of the commentators yet, I can tell you that the Sforno thought that the incense burning here refers to the incense burned upon the altar of incense, not the incense brought into the Holy of Holies.
Second, it should be noted that neither were High Priests. So even if they did enter the Holy of Holies, as per other views as @robev noted, they did so wrongfully.
Third, as @Bpotential pointed out in the comments, in any case they were not commanded to do so, so it was a double-wrong case.