In case of paired programming when more than 1 person is owner or responsible, how can we reflect it on Story/Task? Agile Central does not allow more than one Owner for Story/Task
Unfortunately there is no existing field in Agile Central that would accommodate this. You might try creating a custom multi-value field, although that would require adding each potential owner to that list of values. A text field could be used but that may make filtering and searching difficult. Tags would be another option as well. I'm sorry there just doesn't seem to be a great way to address your requirement at this time.
Custom attribute is fine but not good that everytime we need to update the list. There should have been people picker kind of attribute.
This seems like a reasonable request, I would suggest submitting it as a feature request to ideas.rallydev.com.