Friend of mine asked me this question.

Should I consider registering a second domain name to my site – something like= guitar-lessons.com or guitar-tab.com, For higher SE ranking when people search for those terms. I already have my site registered in all major search engines with a domain name – so will registering it under a second domain be bad or good for my rankings?

Generally speaking, that would be a bad idea. The search engines would treat the two domains as two different sites, and your pagerank would be diluted, etc. If you want to try to rank higher for certain terms, a better idea would be building incoming links with those terms in the link text rather than registering more domains.