Why cant gearbox and transfer box oil mix?

Ive just discovered (on this forum) that both gearbox and transfer box require the same type of oil, MT90 75/90 semi gl4

So why arent they simply allowed to share oil?
If the oil from the gearbox leaked into the transfer box, what harm could it do?
Just curious that's all,

You have 2 seals & a drain cavity between 2 so you never going get fluid from one transfer to other .
In a case where fluids where lost or gained only issue when fluids same spec would be from low oil lack of lubrication & high oil excessive pressure & drag . Design criteria such as optimizing oil levels for each, allowing different spec oils when lsd front diff used, reducing damage & wear in transmission components from fragment debris carried in oil probably part reason for the choice.

Mr B wrote:You have 2 seals & a drain cavity between 2 so you never going get fluid from one transfer to other .
In a case where fluids where lost or gained only issue when fluids same spec would be from low oil lack of lubrication & high oil excessive pressure & drag . Design criteria such as optimizing oil levels for each, allowing different spec oils when lsd front diff used, reducing damage & wear in transmission components from fragment debris carried in oil probably part reason for the choice.

Cheers Mr B, that makes sense to me