where the ui merges the plots just as we want and I was wondering if there is some simple way to do it in the case of all plots.

we can do it for scalars (this is trivial)
We can merge specific plots when they are simple, I think basic histograms.
But for any generic plots we fear the merge will just fail, and this is why it defaults to side by side.

how can I combine two plots in the ui as you mentioned?

The easiest solution is to use, "report_scatter2d", these are specific plot reporting that will be merged when compared
https://clear.ml/docs/latest/docs/guides/reporting/scatter_hist_confusion_mat_reporting#2d-scatter-plots