Possible that they have run out of 845 numbers, but a more probable explanation is that your 2 lines are being services from different "Switches".
If there has been plenty of expansion/development in your area, they would have built a new "sheltered exchange" somewhere close by. So your one line is linked to the "old" exchange and the other to the "new" exchange.
It is also possible that both the exchanges are in the same building.... but improbable.
Run out of capacity on one Benoni exchange and allocated the 2nd as moggie mentioned
to the other telephone switch in the same building or not
Some pics around George of our unprotected Telkom vital Infrastructure "sheltered exchange" located closeby to your neighbourhood
Vicbay Siemens DLU 044-889 + DSLAM - located next N2 National Road
Denneoord installation
In my humble opinion I reckon this is what these cabinets are from left to Right
1) Siemens DLU (extension of the Siemens DE5 EWSD exchange)
2) Street Distribution Cabinet (Blue green) (cable pair connections to Exchange & to subscriber)
3) UMC1000 & DSLAM (extends Fiber to the Curb)
4) SA Post Office mailbox (snailmail)
heh I don't know I applied for the second line in Feb then was told they had a problem getting me a number then a problem with the undeground cable so only time will tell