Firstly, an Mi 8 has 2 Hydraulic tanks, not one as you stated. The Primary and Secondary hydraulic systems are completely independent of each other except that they work through one set of servo actuators - unlike e.g. a Puma which has 2 sets.
Just one tank holding 22l hydraulic fluid. Two independent hydraulic pressure circles, supplied by the single tank. But only one ist the primary system fullfilling all tasks. The secondary system is only a limited "Emergency System".
Limited as example on the following parts:
1. No AP. The AP is normally (1. system) full time on work. Damping the ship reactions and works on condition as a 3 axis AP.
2. Only manual deblocking of the collective. On the primary system the collective is deblocked by the hydraulic system.
3. No angle limiting on the swashplate as normally on the primary system.
Thats the situation on a common 20 years old ex soviet union
Mi-8T with the TV2-117A.
Fully redundant only on the newer Mi-17, Mi171, Mi-8MTV, but this versions are also equipped with other engines, other electronics and APU. Nearly other helicopters...
I have flown the ship first time long before Google. Looking forward to my other errors. To err is human.