Anyone double sewing with light thread ?

[MoMatt]

I have a singer 29 that sews best with 69 size thread.    I am thinking about making a backpack or book bags for my kids that will take a lot of abuse.  Since its so easy to sew back in the same holes with my machine, if I double sew wouldn't I have the advantage equal to stronger lager thread and even much of the look?  A test seam looks pretty good to me.  If I turned the bag inside out after sewing I think it shouldn't matter at all.   

I'm also wondering if I don't need as heavy a thread as I think I do.  Arthur Porter of youtube fame uses size 69 thread for his projects.