Well, my suggestion to most people is to start with Facebook, it helps you work your current Sphere of Influence, and helps solidfy relationships which may go astray.

But Twitter, has this great ability to bring you together with local people, Face to Face, with Tweetups!  That's gold right there, the ability to meet new people outside your current sphere and build a personal offline relationship.  That's huge in my book.

Hi Marcia,

Sometimes it seems like all roads lead to Belleville, I was vacationing in the Caribbean one year and a local vendor had a pictures of himself in front of our city hall!

Anyway, back to your question.

My advice? If your current domain has any front page rankings in the search engines, leave it and use the second domain for a second website which you can also push to the front page of Google.  Or at least use to grab one more PPC spot away from a competitor.

There are only ten spots to grab in the search engines, anything on the second page is useless for traffic, so work to hold as many of the ten you can.  You can also sit on the domain a while and let it age.  Sort of like a fine wine, and then launch the domain and do a 301 redirect to gain back your authority from links going to the old domain.  However, I would not link multiple domains to one site.  There's ways to do it properly, but the last thing you want to to is dilute your link popularity to a site with multiple domains.

I hope that answered your question.