Using 2 GPUs isn't increasing performance

Hello, I recently got a RTX 3070 & I switched it to the main slot on my motherboard with a GTX 1660 on the second slot. But the performance on Blender Classroom scene (with slightly increased samples count) still gives me the same render time as with my single RTX 3070…
I have ticked both GPUs in the preferences & also have a good power supply…

Here are the results of the benchmark :

i7 7700 cpu : 20m 29s

Gtx 1660 (Cuda) : 6m 2.78s
Gtx 1660 (Optix) : 6m 29s

Rtx 3070 (Cuda) : 2m 28.5s
Rtx 3070 (Optix) : 1m 35.5s

Gtx1660 + Rtx3070 (Cuda) : 2m 24.4s
Gtx1660 + Rtx3070 (Optix) : 1m 20.4s
Gtx1660 + Rtx3070 (Cuda) tile size 256 : 1m 34s
Gtx1660 + Rtx3070 (Optix) tile size 256 : 53.6s

What tile size did you do the other tests in?
Have you tested other benchmark scenes to see if they have the same behaviour?
Have you checked in task manager that both GPUs are being 100% utilised?
Is the GTX1660 potentially Vram limited?