Well, actually, it would depend on how the Thunderbolt ports are set up. The 15" MacBook uses two PCI-E buses to drive 4 TB3 ports. The three chips used for TB3 are either:
- one "DP" (Double Port) version that uses a PCIe 3.0 ×4 link to provide two Thunderbolt 3 ports (DSL6540)
- one "SP" (Single Port) version that uses a PCIe 3.0 ×4 link to provide one Thunderbolt 3 port (DSL6340)
- an "LP" (Low Power) version that uses a PCIe 3.0 ×2 link to provide one Thunderbolt 3 port (JHL6240).
The MacBook uses two DP buses. My guess would be that the ports on one side use the same bus, and the ports on the other side use the other bus, just because it makes more sense to run the circuitry that way. Therefore, the two adjacent ports would share bandwidth with each other. TB3 uses an x4 PCI-E 3.0 bus, therefore, technically, you'd be splitting up around 4 GB/sec or 32 Gb/sec (PCI-E 3.0 x4 theoretical bandwidth limit of 985 MB/sec/lane) of bandwidth over two TB3 ports. Hence why I said that the two ports would be sharing bandwidth and that they probably just put two ports on that hub to get 14 Watts of power to the hub and add more physical stability. Does that make more sense now?
As a side note, to add to the confusion, some manufacturers use the LP version of the TB3 bus, which only yields a 16 Gb/sec bandwidth, far from the 40 Gb/sec you would assume it would have!