i can't defend doing one thing at a time because i've always been involved in parallel projects.
if you focus on back-end you'll pick up minimum viable frontend chops along the way. with free resources like Orange and Refactoring UI you don't need to "be a designer" to build functional, decent looking products.
and hey -- if the only thing between your product being good and being great is design, just hire a kickass designer and implement their work as a final polish. i've done it many times.
to learn to code with me, go here.