Since all trains are all seats reserved from DC to New York, you will have a seat. As for whether you will have two seats together, that is a different issue, If you take an early enough train in the morning on Friday you probably stand a slightly better chance of getting seats together. Of course in all cases if you stand in the boarding line near the front you will get seats together since seats are first come first served when boarding. This means getting to the boarding gate as soon as possible after you know which one it is.
On weekend days you are more likely to get to sit together.
If you are leaving from DC, even during busy weekday evenings, you can usually find two seats together, as long as you are willing to hoof it to the very front two cars of the train. And do the walking on the platform, as you will be held up by passengers selecting seats, trying to hoist luggage into the overhead racks, etc.if you try to walk up through the train. The last car (closest to the gates in Washington) is business class, the car in front is the quiet car, there's another coach before the cafe car, and then there are three or four coaches in front of the cafe car.
If you have any excuse to get into the priority boarding line (i.e. age, presence of small kids, service member in uniform, AGR status) you will have the first chance to get the seats. To be sure you get 2 seats together, the $5 or so tip you give to the redcap is well worth it, as you'll get boarded before any one else, even those with priority boarding.
It should be noted that, strictly speaking, this applies only to the Northeast Regionals that originate in Washington. There are a bunch of Northeast Regionals that originate in various parts of Virginia, and those trains will already have passengers even if you are the absolute first to board. I would say that in those cases, to ensure having two seats together, the use of a redcap is a really good idea.