The laws in the United States vary by state. In Wisconsin, the law on homicide by negligent operation of vehicle says:
Whoever causes the death of another human being by the negligent operation or handling of a vehicle is guilty of a Class G felony.
However, he also fled the scene. Under Wisconsin law:
The operator of a vehicle involved in an accident shall reasonably investigate what was struck and if the operator knows or has reason to know that the accident resulted in injury or death of a person or in damage to a vehicle that is driven or attended by a person, the operator shall stop the vehicle he or she is operating as close to the scene of the accident as possible and remain at the scene of the accident...
Fleeing the scene of an accident is a misdemeanor if there were no serious injuries. However, if the accident involved death, then it becomes a class D felony.
What is the maximum sentence he could get?
A class G felony is punishable by a fine not to exceed $25,000 or imprisonment not to exceed 10 years, or both.
A class D felony is punishable by a fine not to exceed $100,000 or imprisonment not to exceed 25 years, or both. As you can see, this is actually a longer sentence for fleeing than for causing the two deaths in the first place.
However, it turns out that they can't actually sentence them to prison for that long. The number listed above is the combination of years in prison and extended supervision that he may be sentenced to. Here is what the prison portion of that sentence is allowed to be:
For a Class G felony, the term of confinement in prison may not exceed 5 years.
For a Class D felony, the term of confinement in prison may not exceed 15 years.
As far as I can tell, Wisconsin judges are free to impose the sentences concurrently or consecutively.
So, for 2 counts of homicide by negligent operation of vehicle plus 1 count of hit and run involving death, the maximum prison time could be 5 + 5 + 15 = 25 years, followed by 5 + 5 + 10 = 20 years of extended supervision, along with a fine of $25,000 + $25,000 + $100,000 = $150,000.
You say he also fled the country. Assuming he did this to avoid prosecution, this would be a federal crime punishable by up to 5 years in federal prison, in addition to the state charges.