Firstly thanks for your positive comments!
To answer your question from a tax point of view, you can do all the things that you want to do with your existing LLC's. The advantage to you of using your existing LLC is that you would save an additional cost of $500 for incorporating a new entity as well as don't forget you will only have to file a Single LLC return instead of 2 returns. Again, you will save Washington State (or DC) annual filing fees for 1 entity. Remember you have to pay a minimum filing fee for every entity that you intend to file!
So, the advantages of using the old entity are usually financial in nature and stem from the fact that you would save tax return fees and annual filing fees for 1 entity.
I dont see any disadvantage as such, because if you can always use a DBA to describe your business to customers and not the actual LLC name, and that is permitted!
Now for your second concern! Should you use a different LLC for a different business altogether. Here you have an entirely new physical location. Clearly,this is different situation altogether from the earlier discussion. In my CPA practice of over 20 years, i have seen that businessman have been advised to form a new entity to conduct a new business at a different location.
Here the business risks and liability issues override the tax filing savings etc as described above. The usual practice is to form another LLC to conduct the other business. Once again, this simply means that you want to completely separate the accounting and tax activities of each entity for reporting purposes. This is will definitely be more expensive than consolidating the 2 businesses into one LLC activity.
But, my preference is to separate the businesses into 2 separate LLC's.