Theoretically if you keep the size of the entrance pupil and field of view the same then you will capture the same total amount of light regardless of the format.

If your medium format sensor in 1.6 times larger (which is the upper end available today, the Leica S2 you mention is only 1.25 times larger), then to match your 35mm DSLR and 85mm f/1.2 lens would require a 135mm f/1.9 medium format lens.

f/1.9 lenses are rare in medium format (mainly for weight reasons), the only current(ish) lens I know of is the Mamiya 80 f/1.9 - this lens can compete (in terms of total light gathered) with a 50 f/1.2 on a DSLR.

However even though the 80 f/1.9 lens transmits same total amount of light as a 50mm f/1.2 that doesn't mean the medium format system with this lens is as good in low light - this is due to the sensor being less efficient.

Medium format digital backs are most commonly used in landscape, architectural or studio photography where a certain amount of light is guaranteed by the use of a tripod or large artificial light sources. The sensors are optimised for size, producing accurate colours and fine tonal graduations as opposed to offering high sensitivity.

For this reason nothing commercially available today can match the low light performance of a 35mm DSLR.