That squab looks about at the 3 week mark, almost fully feathered. In about a week, aside from its ceres and head size, it will look just like its parents. Around this age, they still want to be brooded / sat on, but they are getting too big, but mom and dad will still sit on them for periods of time of the day, mostly at night. When they do sit on them when they are large like this, it is distinctly possible that only one can fit under the parent. As such, I think this is pretty normal, at least as far as I can tell from the picture. If you watch them for a while, I'll bet you will notice the parents will leave them alone in the nest for periods of time. Pigeons are very good parents (both the mom and dad care for them, you may not notice, as only one will tend to them at a time usually). I do not think you need to intervene. This is all part of the process of getting them weaned and ready for the real world.
What you could do to help them, however, is to take steps (if you can) to ensure they are not killed by predators and such, as I notice they are on the ground.
You can provide food and water for mom and dad, might make things easier for them in these winter months (most ferals avoid breeding this time of year), but it is likely not necessary, pigeons are usually good at determining when is an appropriate time to lay and hatch young (i.e., when they'll have enough food and water, etc). If you offer water, just make sure it's not too deep of a vessel such that the squabs could drown in it.
Young pigeons, even after being weaned, have a strong desire to be close to their parents. Even after being taught to forage and eat grains, they'll still beg to be mouth fed by dad. When squabs are about 4 weeks old the mom will start laying again close to the old nest or perhaps in it. I recently caught one of my 4 week old squabs cuddling in the same nest box as his mom as she sat on new eggs. I was surprised she didn't kick him out (like parents telling their kids they're too old to come lay in bed with them lol), she just let him cuddle with her in the nest bowl as she sat on new eggs, it was adorable.