First, to your specific question. Dahlias as usually started from a single tuber. That means that it is a tuber with a neck and crown material from an original clump. If you have clumps now, you can see the crowns at the top of each tuber neck where the old stem is. They appear as slightly swollen portions of the neck. This is where the "eyes" form, and eyes become sprouts, which become dahlias. Each tuber can have many eyes, and produce many sprouts. So each clump has the potential to produce many single dahlia plants that will all bloom this season like your clump did last year.

If your single tubers don’t have necks, or don’t have crown material around the neck or the neck is broken, then they won’t produce anything but roots. A dahlia that produces tons of roots will be totally without flowers if it doesn’t have a crown that produces eyes.

So let’s assume they do have intact necks, crowns, and eyes are forming…

Putting them together with the clump in the pot is no problem. Every eye in the pot will likely produce a sprout, whether it’s part of the clump or not.

The next question is what are you trying to achieve? If you want to grow your dahlia all season in a pot, then you’re going to want to pick one main sprout and cut off the others once the main sprout has demonstrated it is growing well (I don’t know, say when it gets 12" tall or to the point where you’d top it.)

Conversely, if your objective is to simply get started early so your clump will bloom sooner in the summer, then the same advice is probably good.

Finally, if your goal is to get more plants of the same variety and plant the clump in the garden too, then you could take cuttings as the sprouts come up (say, leaving the first sprout intact to become the main stem of the clump when you transplant.) The cuttings will each form a full plant, likely bloom as good as the clump, and should create tubers of its own this year.

Now, one other way of thinking about what you’re trying is to think that you’ve heard of "pot tubers" and are trying to create them. Pot tubers are cuttings that are grown in 4" pots and left in those pots all season, putting the pot into the ground possibly even next to clumps planted in the ground directly. Pot tubers are nice because their compact and easy to lift in the fall. I’ve been assured they grow every bit as good as "field tubers", which are tubers grown directly in the ground with no restriction.

Pot tubers are started from cuttings so the tubers can grow in the restricted environment. This results in smaller tubers (typically) and more of them (as opposed to a massive tuber in the pot.)

If pot tubers are your objective, then planting the clump (and well formed loose tubers) in a container will get you the sprouts to cut off into cuttings. Plant the cuttings in 4" pots and restrict the water and give them 14 hours of light a day. This will stimulate root growth. Eventually you’ll see the leaves and stem continuing to grow, which should indicate good root growth. These can go into the ground (in their 4" plastic pots) the same time you put your clumps in the ground. Stop taking cuttings about 6 weeks before you plan to plant the clump.

I’ve probably gone way off topic; I hope I did answer your question in there somewhere though.

FWIW, I started 280+ tubers in mid January and have so far taken ~100 cuttings from the 45 tubers that sprouted.

Cheers,
Russ