It's better to kill one.. preferably the weaker if there is a difference. The roots will compete with each other and one will likely dominate the other.
if you use size-appropriate pots, the cost of adding an extra seedling pot is near zero. Plus, irrigation is 10x easier when you don't use an oversized pot for a tiny plant. It's difficult to water properly. Even if you water a small circle around it and gradually increase, it still results in less than idea root growth. Roots won't grow into the dry areas. Any zone on the edge of that will continually deposit nutrients and over time 'could' cause some high concentration or uneven zones of nutrients, which is never a good idea. Sure, most times you can avoid problems, but it's inviteing problems to occur, eventually, that would be otherwise avoided. Use size-appropriate pots and stick to orthodox irrigation methods.
orthodox
1) fully saturate - this should never cause a problem. Also, 10% runoff religiosly if it is a "soilless" grow.
2) wait for appropriate dryback and repeat.
don't make watering complicated. These are best practices for potted plants and marijuana is not a unique case in this regard.