It's not unusual for a nation to have multiple police forces with overlapping jurisdictions. And I don't see it as a problem in and of itself. In Ottawa, Canada, for example, because it is the national capital there are three police forces with jurisdiction within the city: the Royal Canadian Mounted Police (the federal gendarmerie force), the Ontario Provincial Police, and the Ottawa Police Service. As far as I know, and I live in Ottawa, it works out well.
But back to France. I imagine that IT costs are a small portion of the gendarmerie or police's budget. Most of it would be taken up by wages for the officers and staff and maintenance of equipment and facilities. Merging France's two police forces would do little to reduce those costs, since the combined organization would have the union of the two organizations' duties.
That can be a bad thing as well if those tools are chosen poorly. By diversifying you don't necessarily have the same weaknesses across all your components - and other parts of the group can copy things that work when someone tries something new out.
But back to France. I imagine that IT costs are a small portion of the gendarmerie or police's budget. Most of it would be taken up by wages for the officers and staff and maintenance of equipment and facilities. Merging France's two police forces would do little to reduce those costs, since the combined organization would have the union of the two organizations' duties.