One of the common features of the Italic and Celtic branches is the merger of perfect and aorist. So, in the surviving "perfect" forms we find a mixture of old aorist stems and old perfect stems.
Here I want to know, how parallel this merger was. Of course, for such kind of syncretism some roughness and irregularities are expected, one stem wins over the other in one dialect, but not in the other. However, there should be a large amount of stems where the choices in Italic and Celtic are the same.
Are there some quantitative studies on this topics, and what are the main results? Can a Proto-Italo-Celtic perfect be reconstructed for a large portion of verbs? Or, to the contrary, is it a convergent process happening independently twice in the history of the languages?