12 days isnt that big of a deal depending on if they are the same strain.
Unless they are far enough apart that the roots dont intertwine, but if one finishes in 10 weeks and the other in 8 weeks and depending on how thats arranged (8 week is the 12 day head starter) you could have rotting roots in a res for more than 3 weeks. On the other hand if the 12 day head starter is the 10 week and the day oner is 8 weeks then you're golden.