I believe you are misinterpreting the diagram. This is a rather simple way of trying to show how the RF is used over time to clear the frequency, establish timing, and set any parameters necessary for the information to be transmitted.
802.11ac has to remain backwards compatible with previous 802.11 standards so they can see and avoid transmitting on the RF for the time specified. While 802.11n offered three PHY formats (legacy, mixed and green field modes), 802.11ac was simplified and only offers one.
The legacy training fields and signal fields need to be sent in a format that will be recognized by legacy devices (i.e. 802.11a and 802.11n) which means that they can only occupy a 20MHz channel. As the legacy fields can not set the VHT parameters, the initial VHT fields must follow this as well.
After that point, the rest of the VHT communication can take up the entire channel width if it is larger than the initial 20MHz.
But consider, what happens if you only transmit those initial fields on the first 20MHz but your channel is 40MHz (or larger)? Legacy devices operating on those channels will not receive the information they need to avoid using the channel for the time specified and they could then create a collision outside of that first 20MHz.
So to avoid this potential problem, the same initial fields are duplicated and sent out on the second (and successive) 20MHz channels as well. It is only once these initial fields are sent and the VHT transmission starts that it can utilize the full width of the 40MHz (or larger) channel.
If it helps, think of the diagram as illustrating lanes on a road. If you want to send an "escort vehicle" ahead to clear the route, you would need to send one down all the lanes at the same time to ensure they were clear and that no other vehicle merged into the "gap."
For bonus points, I should point out that 802.11 doesn't really transmit bits, it transmits symbols. The number of bits transmitted per symbol is determined by the modulation and coding rate. So in a sense, bits are always stacked on top of each other. For example, 802.11a BPSK R=1/2 (6 Mbps data rate) can transmit 24 bits per symbol.