Dear Maximilian,
"half-duplex" means that the memory bus can either be used to read or to write a cache line, but not both at the same time. However, it can alternate as required between the directions. Hence, the overall memory bandwidth is 110 GB/s, independent of the RD/WR ratio. You can use the 110 GB/s for read-only, for write-only, for copy (where 2/3 will be RD and 1/3 will be WR if write-allocate applies), etc.
Hope that helps,
Georg.