Is there any research going on in producing fusion on a single pair of nuclei, sequentially?

Yes, there have been numerous attempts to use individual accelerator-driven ions to create fusion.

The first was by Rutherford's team, Oliphant in particular. In 1934 they demonstrated fusion by accelerating protons and firing them into thin metal foils that had various elements diffused into them. You can read a very readable paper on the topic here.

Through these experiments they were able to calculate the cross-section of the D-Li/D-D reaction, and it demonstrated that the scattering potential was many orders of magnitude higher. In other words, when fired at each other like this, in the vast majority of cases the D's will simply reflect off each other before they fuse. It is relatively trivial to show that the amount of energy lost through these "missed connections" is many many times higher than the amount of energy released by the rare fusion reactions.

That does not mean this technique is not useful! It has been used repeatedly since the 1930s to measure the cross sections of many other reactions. But it does mean it is not useful as a power-generating system.

And this is why most research focuses on "hot fusion". Very simply, by heating up the fuel it's always bumping into each other at high energy (that's what a gas is) so instead of having one chance per ion to collide and react, it has many (we hope!). Unless you get thousands of such attempts, you're on the wrong side of the energy curve.

However, that has not stopped some from coming up with clever arrangements that try to do this in other ways.

One of the first was the fusor, or more generally IEC. These systems use arrangements of wires to produce was is essentially a spherical accelerator so that an extended ball of fuel is being accelerated towards the center of the chamber. This alone increases the chance of collision by increasing the density as it approaches. However, if an ion does miss a collision, when it exits the other side of the center it is accelerated back in, repeatedly travelling into the reaction area. However, in practice the fuel hitting the wires of the reactor removes energy very quickly.

A modified version of the fusor is the polywell. This uses an arrangement of magnets to pull the electrons into a layer that simulates the electrodes without any physical barrier to hit like the wires. There was great hope for this approach, but there are more modern reasons to believe these cannot work even in theory (see end).

Another, perhaps more direct, attempt is the migma machine. This used a very clever system to store the particles in circular orbits around a common center, sort of like a collection of comets around the sun. So for a short period on each orbit they pass by many other particles in their orbits. Due to the arrangement of the orbits, the ions are going in opposite directions, and have several chances to collide. This system worked, but it was pointed out that purely theoretical reasons suggested it's possible ion density was very low and that it could not be a practical machine. Research on this approach ended in the early 1980s IIRC.

More recently, Tri-Alpha Energy has developed a new design that has some parallels to the migma concept. Instead of an external storage system, it instead fires the ions into a plasma arrangement known as an FRC. I believe the idea is that the FRC's internal magnetic fields basically replicate the storage system of the migma, but I'm not sure I completely understand the design, and I cannot find any recent and clear description of the system.

All of this was upset beginning in the late 1990s, when a series of new studies on the theoretical performance of such systems appears to put the kibosh on any of these approaches. In any system where the fuel is not maxwellian, the scattering events see by the ions and their electrons rapidly cools the particles. This suggests they will all lose energy faster than they can produce it. TAE has claimed their system is maxwellian and does not suffer from this problem, but when I read it I am not so sure, but I simply don't know enough about their layout to make a good judgement.