For the first iteration of the Renault engine in early 2014, they tried to renounce to use a wastegate. They tried to control the pressure only with MGU-H, and fitted a big resistor to dump energy into it if the energy store was already full.
It didn't work, and the resistor got too hot. They didn't foresee that they had to limit MAP during transients so frequently.
A former Lotus employee somewhere on another forum wrote pretty detailed info about those first huge problems in testing at that time.
Maybe they still didn't give up to use as few as possible the wastegate (obvious as no one wants to dump energy out through a wastegate...), and can get off with only one bank for simplicity and weight reasons?