Arjuna also was trained by a lot of people.

Uttara asked how Arjuna alone will fight against a lot of Kauravas.

And Uttara said, 'Thou, O best of Pandavas, art alone. These mighty car-warriors are many. How wilt thou vanquish in battle all these that are skilled in every kind of weapon? Thou, O son of Kunti, art without a follower, while the Kauravas have many. It is for this, O thou of mighty arms, that I stay beside thee, stricken with fear.'

Arjuna laughed and he said that he should have any fear.

Bursting out into loud laughter, Partha said unto him, 'Be not afraid, O hero, what friendly follower had I while fighting with the mighty Gandharvas on the occasion of the Ghoshayatra? Who was my ally while engaged in the terrific conflict at Khandava against so many celestials and Danavas? Who was my ally when I fought, on behalf of the lord of the celestials against the mighty Nivatakavachas and the Paulomas! And who was my ally, O child, while I encountered in battle innumerable kings at the Swayamvara to the princess of Panchala?

He then tells that he was trained in arms and using weapons by various people and asked Uttara why he should not fight the Kuru army?

Trained in arms by the preceptor Drona, by Sakra, and Vaisravana, and Yama, and Varuna, and Agni, and Kripa, and Krishna of Madhu's race, and by the wielder of the Pinaka (Siva), why shall I not fight with these? Drive thou my car speedily, and let thy heart's fever be dispelled.'"

*Note the bolded words where Arjuna said that he was trained in arms by Drona, Indra who is Sakra, Vaisravana, Yama, Varuna, Agni, Kripa, Krishna, and Shiva. This means that you could have more than one guru.

Same is the case with Pitamaha Bhishma, the grandfather of the Pandavas and Kauravas also.

https://www.sacred-texts.com/hin/m01/m01101.htm

Endued with superior intelligence, he has studied with Vasishtha the entire Vedas with their branches. Skilled in all weapons and a mighty bowman, he is like Indra in battle. And, O Bharata, both the gods and the Asuras look upon him with favour. Whatever branches of knowledge are known to Usanas, this one knoweth completely. And so is he the master of all those Sastras that the son of Angiras (Vrihaspati) adored by the gods and the Asuras, knoweth. And all the weapons known to the powerful and invincible Rama, the son of Jamadagni are known to this thy illustrious son of mighty arms. O king of superior courage, take this thy own heroic child given unto thee by me. He is a mighty bowman and conversant with the interpretation of all treatises on the duties of a king.' Thus commanded by Ganga, Santanu took his child resembling the Sun himself in glory and returned to his capital.

Ganga tells Shantanu how Brihaspati, Parashurama,Usanas, and Vashishta trained Bhishma.

Arjuna also was trained by a lot of people.

Uttara asked how Arjuna alone will fight against a lot of Kauravas.

And Uttara said, 'Thou, O best of Pandavas, art alone. These mighty car-warriors are many. How wilt thou vanquish in battle all these that are skilled in every kind of weapon? Thou, O son of Kunti, art without a follower, while the Kauravas have many. It is for this, O thou of mighty arms, that I stay beside thee, stricken with fear.'

Arjuna laughed and he said that he should have any fear.

Bursting out into loud laughter, Partha said unto him, 'Be not afraid, O hero, what friendly follower had I while fighting with the mighty Gandharvas on the occasion of the Ghoshayatra? Who was my ally while engaged in the terrific conflict at Khandava against so many celestials and Danavas? Who was my ally when I fought, on behalf of the lord of the celestials against the mighty Nivatakavachas and the Paulomas! And who was my ally, O child, while I encountered in battle innumerable kings at the Swayamvara to the princess of Panchala?

He then tells that he was trained in arms and using weapons by various people and asked Uttara why he should not fight the Kuru army?

Trained in arms by the preceptor Drona, by Sakra, and Vaisravana, and Yama, and Varuna, and Agni, and Kripa, and Krishna of Madhu's race, and by the wielder of the Pinaka (Siva), why shall I not fight with these? Drive thou my car speedily, and let thy heart's fever be dispelled.'"

*Note the bolded words where Arjuna said that he was trained in arms by Drona, Indra who is Sakra, Vaisravana, Yama, Varuna, Agni, Kripa, Krishna, and Shiva. This means that you could have more than one guru.

Same is the case with Pitamaha Bhishma, the grandfather of the Pandavas and Kauravas also.

https://www.sacred-texts.com/hin/m01/m01101.htm

Endued with superior intelligence, he has studied with Vasishtha the entire Vedas with their branches. Skilled in all weapons and a mighty bowman, he is like Indra in battle. And, O Bharata, both the gods and the Asuras look upon him with favour. Whatever branches of knowledge are known to Usanas, this one knoweth completely. And so is he the master of all those Sastras that the son of Angiras (Vrihaspati) adored by the gods and the Asuras, knoweth. And all the weapons known to the powerful and invincible Rama, the son of Jamadagni are known to this thy illustrious son of mighty arms. O king of superior courage, take this thy own heroic child given unto thee by me. He is a mighty bowman and conversant with the interpretation of all treatises on the duties of a king.' Thus commanded by Ganga, Santanu took his child resembling the Sun himself in glory and returned to his capital.

Ganga tells Shantanu how Brihaspati, Parashurama,Usanas, and Vashishta trained Bhishma.

--

https://sacred-texts.com/hin/m03/m03307.htm

Karna also was a student of Drona, Kripa, and Rama of Bhrigu's race which is Parashurama.

And there Karna put up with Drona, for the purpose of learning arms. And that powerful youth contracted a friendship with Duryodhana. And having acquired all the four kinds of weapons from Drona, Kripa, and Rama, he became famous in the world as a mighty bowman.