It might help to take a step back and ask why first. Are you working on a project that requires both?
If you just want mercenary ability with C and ASM, you'll have no problem finding the resources - just a matter of finding the best book. I'm sure you'll get plenty of recommendations here.
It sounds you want to enrich your general programming knowledge though. In that case I'd recommend learning computer architecture concepts - once you have that down, learning C and ASM will be a pretty transparent process. In fact, I'd recommend it even if you just need it to hack on something specific. I took an architecture course at uni that used this book, and it was pretty decent: http://www.amazon.com/Computer-Organization-Design-Fourth-Ar...