I agree that engineering salaries are much higher in the US than in Europe and other non-US countries, however it's worth considering that there are additional expenses to hiring in other countries. My french salary is ~30-40% lower than I was making in the US. However, my cost to my employer is nearly as high. Employer taxes are much higher, my company is required to reimburse my transit and all-but-obligated to cover lunch as well. Certainly, many US software companies do some/all of that, but not all.
And beyond those pure costs, there are more liabilities to the employer. For instance, if they want to fire me, they're legally on the hook for four months of severance. And, I get ~7 weeks of combined vacation time and "comp time" (based on the fact that I work more than 35 hours a week). In the US, I got 2-3 weeks.
I don't have exact numbers, and I don't disagree that there are potentially savings to be had, but I don't think it's nearly as clear cut that you could get anything close to twice as many developers.