It's generally okay to keep two male zebra finches together, especially if they are raised together from a young age or introduced properly. Zebra finches are social birds and often do well in pairs or groups. Keeping two males can prevent issues related to breeding and nesting, such as aggression or territorial disputes that might occur if both males and females are housed together. However, ensure that the environment is enriching and spacious enough to accommodate their needs. This includes providing multiple perches, toys, and food stations to prevent any potential aggression over resources. Regular interaction and monitoring are important to ensure they are getting along well. If you notice any signs of stress or aggression, you might need to consider different arrangements or consult with a veterinarian or an avian specialist.