You cannot 'fuse' two Raspberry Pis in the way you're thinking—it's simply not possible.
Computer clusters do exist, and indeed, some people do make clusters of Raspberry Pis. But two slow computers don't make one fast computer, generally speaking—clusters of computers have quite a narrow use case. It would probably be a waste of your time (and money) to try and build a cluster to make video playback faster, so I wouldn't advise it1.
Clusters work well for large computations that can be done in parallel. Imagine you need to cut 1,000 pages in half with some scissors. It'll take you a long time to do, but you could give 100 pages to 10 friends and they'd do it in a tenth of the time... but, each individual page doesn't get cut any faster, and you have to co-ordinate splitting the jobs and re-assembling the pile at the end which takes a little longer. Cluster computing is largely the same, so if your task isn't something that can be parallelised, then you'll see little benefit.
If the video is stuttering, you really need someone who can cut their pages in half faster, not more people—ten people trying to cut one piece of paper takes just as long. In the same vein, you need a faster computer, not more slow Pis in a cluster.
If you're keen to stick with the Pi, you should really address why the video playback is slow—try to figure out what is causing the slowdown, and perhaps look around this site and ask for more help if you need it.
See also: Why Use a Cluster? on eTutorials.org, which explains what use a cluster is and when you might use them.
1That said, clusters are very interesting, and if you do have a use case for them after learning more, do consider trying to make one!