An LLC (or a person, for that matter) can do business under more than one dba. More than one business can be linked in the sense of being part of the same LLC and sharing the limitation of liability.
See the form and instructions for LA County linked below.
As I mentioned in my answer to your previous question, having both businesses under the same LLC umbrella means that Website X's profits will be offset against Website Y's losses.
If you add more websites/businesses to the LLC, yes, you'd do another dba. And again, the losses of one business would affect the profits of the other(s).
When you dissolve your VA LLC, you'll have to transfer Website X's assets to the new LLC.
Hire a business lawyer to organize your business properly and make sure you've handled the requisite business licenses, website disclaimers, disclosures, insurance, IP issues, etc.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
In response to your additional question, you should file the Fictitious Business Name Statement (also known as a DBA) in the county in which the business will primarily be operating. If you operate a Web business from your home, that would be the county in which you reside (which appears to be Los Angeles from your post).
LLCs are not difficult to form or maintain. If you're concerned about liability, note that if both sites are operated by the same legal entity, an expensive claim against one Web business could eat up the assets of the other, presumably successful Web business. The safest structure would be to put the businesses in two separate LLCs, viewing the $800 annual franchise tax as an insurance premium of sorts. (Insurance coverage, if available on reasonable terms, is also an option.)
As indicated before, there is no bright line rule per se for the separation or segregation of a website operation under a DBA or separate entity (e.g., LLC). However, as I often advise my clients, the question should not be "does my website need a separate dba, etc.?," but rather, "why doesn't it require a separate dba, etc.?" The reason being is that, the limited liability protection of the initial entity (e.g., LLC, etc.) will only extend to the operation that it can clearly be associated with. And secondly, the other websites may have unique liabilities and revenue streams that should not be commingled under one entity's roof.
Even if these other websites are not revenue generators, they may be liability generators (e.g., a free web forum that routinely deals with user generated content and third party copyright is prone to infringement claims), it might still make sense to house them separately. Finally, with regard to dba's and websites, my advice is to skip the DBA if you decide to segregate the website, and instead get an entity. The reason being is that a DBA is in many states only good for a particular county or set of counties, while the site is presumably accessible globally.
You do not make clear if you already have an entity (perhaps you are referring to it with "ABC Company?"). However, if you do not already, I strongly suggest that you get the process in motion.
And as a final bit of advice, it might make sense to have strongly drafted terms of service agreements for the sites, in addition to, or at least in lieu of, getting separate entities for each. These agreements can offset many of the above liabilities and also make clear to the user that he/she is dealing with a particular entity.
You should consult with a local attorney familiar with these issues to guide you through the process. And I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.