Barom-1

From ebookrenta:

Takeshi and Kentaro are two friends who have been together since they were infants. They were born in the same hospital on the same day, have shared every class in school and even vie over the same girl. However, soon the two friends become closer than they'd like when they discover they are the Chosen Ones that will defeat the powers of an evil being and his minions who are currently wreaking havoc on mankind. Together, the two merge and become a super fighter known as Barom One, capable of taking down monsters with ease. Can Takeshi and Kentaro band together to save the world?

Serialized from 1970-1971 in Kodansha's Weekly Bokura Magazine. It was collected into two tankobons.