It would be a vast improvement in terms of both overall stability and better management of the provinces.
I am not that confident. Look at Varro, the governor of Sicilia, which is the best documented case, because Cicero was the lawyer of the sicilian cities. I am afraid, that very often not that much would change. Varro had a quaestor and legates. All nominated by the senate. The quaestor even with an independent mandate. Nevertheless, they all worked together harmoniously in order to exploit the province. Even if the senate appoints 2 colleagues from 2 opposing factions intentionally, the chances are high, that they work together, if it comes to exploitation. The senate knew, that the provinces are exploited, but did not care. Most impeachments in Rome failed, due to the aristocratic jury.
Collegial provincial magistratures would help. No doubt about that. But it is just a first step and one of many needed measures. Exploitation did not stop until the princeps. Because the princeps was not interested in a bunch of greedy aristocrats ruining his tax-base in the long run.
Of course, it required the Romans to bother about actually administering the provinces, as opposed to just looting them.
Correct. However, they could have easily increased the workload , if they abolish the system of tax farming by roman
publicani and implement direct taxing of cities like Julius Caesar introduced for Asia. This alone, helped a lot.
There was also room for improvement in the department of inner security. The romans just took care about revolts or conflicts between cities. Sometimes piracy or big brigand bands. Their support for the the cities, which were also responsible for huge adjacent counties, often without any police forces, was rather poor and did perhaps not happen before the 2nd century AD (beneficarius, centurio regionarius).
Same with iurisdiction. A roman governor travelled once a year from one juidicial district (
conventus) to the next (e.g. 3 in Lusitania, 6-8 in Asia). If they do that just twice a year the workload increases significantly. And there would have been no time for a military campaign anymore. So a split of tasks or delegation / buerocratization becomes a must do.
Also the auditing of cities, was rather poor. The romans usually waited until a city was bankrupt by corrupt or incompetent local magistrates until they took action.
Furthermore, there was no role model for that. A consul or praetor in Italy had both civilian and military authority.
Unfortunately, this is true. The romans had no intention to divide powers, or implement a hierachy with clear delegation. Well, they did it to a certain extent functionally. A consul in Rome has not that many civil duties. It was more or less, just to run meetings of the senate, of the assemblies and manage the elections. While the other consul out of Rome was leading an army. Iurisdiction was mainly done by the praetors, finance and documentation by the queastors and a lot of administrative tasks by the aediles. And none of them reported to the consuls! So a functional split is possible in the roman world, which does not neccesarily mean a division of power in the modern sense.
So I could imagine, 2 pro-praetors in a province. One taking care about iurisdiction, auditing cities and taxation. While the other is leading the army in the province (external and internal security). Actually the romans did something similar. The pro-praetor usually used his quaestor and legates to split the work in every department. But it was up to him how to do it. It was not regulated by the senate.
The model they had included the possibility of, say, two magistrates with equal powers in every province (perhaps this starts in Spain; rather than dividing it into two provinces, they create one big-ass one with two proconsuls), but separating a military and civil sphere in a collegial magistrature was not done in Republican Rome.
I doubt this would work. The reason for smaller provinces was also to reduce travel time every year to the main cities (conventus). And Spain is a very bad example. In imperial times, the Tarraconensis was already too big. So the legatus legionis in Asturia always performed all tasks of the governor in Tarraco, which did not happen to such an extent in other provinces. Later a procurator did the job in Asturia on behalf of the governor, even if that was definately not his job and he was not reporting to him officially. So clearly this was exception handling. Distance had a meaning in ancient times.
But I agree, the romans would perhaps start with 2 pro-praetors, both with full power even in a smaller province like Sicilia, if just the workload would have been higher. But that needs a different roman understanding of a governors duties.