Speed decreases with the number of DIMMs per channel. You get the full 2666MHz with a single DIMM per channel, 2400 for 2, 2133 for 4. However, the controller for each processor has 8 channels, and the T2 has 8 DIMM slots per CPU, so I don't think you're going to have a problem as long as you have both CPU sockets populated.
Here's what lshw reports on my 64GB dual-8 Talos II (the workstation I'm typing on):
*-memory
description: System memory
physical id: 0
size: 64GiB
*-bank:0
description: RDIMM DDR4 2666 MHz (0.4ns)
product: 36ASF2G72PZ-2G6F1
vendor: Micron Technology
physical id: 0
version: 0000,19 20,25
serial: 0xf054945f
slot: UOPWR.A100059-Node0-DIMM2
size: 16GiB
clock: 2666MHz (0.4ns)
capabilities: ecc spd-1.1
configuration: errordetection=ecc rank=2
*-bank:1
description: RDIMM DDR4 2666 MHz (0.4ns)
product: 36ASF2G72PZ-2G6F1
vendor: Micron Technology
physical id: 1
version: 0000,19 07,0F
serial: 0x20ab81d0
slot: UOPWR.A100059-Node0-DIMM6
size: 16GiB
clock: 2666MHz (0.4ns)
capabilities: ecc spd-1.1
configuration: errordetection=ecc rank=2
*-bank:2
description: RDIMM DDR4 2666 MHz (0.4ns)
product: 36ASF2G72PZ-2G6F1
vendor: Micron Technology
physical id: 2
version: 0000,19 20,25
serial: 0xf054948a
slot: UOPWR.A100059-Node0-DIMM10
size: 16GiB
clock: 2666MHz (0.4ns)
capabilities: ecc spd-1.1
configuration: errordetection=ecc rank=2
*-bank:3
description: RDIMM DDR4 2666 MHz (0.4ns)
product: 36ASF2G72PZ-2G6F1
vendor: Micron Technology
physical id: 3
version: 0000,19 07,0F
serial: 0x20abb8b7
slot: UOPWR.A100059-Node0-DIMM14
size: 16GiB
clock: 2666MHz (0.4ns)
capabilities: ecc spd-1.1
configuration: errordetection=ecc rank=2