They are working both apps at the same time, picking rides from the first that pops up. I don't see how hourly rates would work with that unless they timeslot every hour to one company, which is not how it currently works.
But that doesn’t matter because you would calculate minimum wage by taking the amount of time worked with the income and then computing the difference between the hourly rate.
The question is whether you'll be paid by Uber while completing a ride from Lyft, and if the answer is "no" how that'll happen technically. Or perhaps you don't get paid for idle time - I'm not quite clear on that point.