[QUESTION] Two different PCIe graphics cards: is it useful or not?

I’m learning tensorflow, so sorry if my question is too stupid.

I’ve just bought a RTX 3070, upgrading from my old GTX 970, both for gaming and for using with tensorflow.

If I use the two GPUs, my motherboard will “split” the speed of the PCIe3 slots from 16x to 8x. So my question is: is it worth it to keep the GTX 970 along with the RTX 3070 to send a part of the tensor calculation to the 970 using Strategy? Can the 970 give me more speed for training NNs, or will the gains be negligible? Do you guys think I should keep just the RTX 3070 on the motherboard?

My PSU is powerful enough for both (1000 watts), so the question here is just about the usefulness of the GTX 970 as a second gpu for NN training in tensorflow, since it has some CUDA cores that (I suppose) could potentially be used alongside the 3070.

What do you guys think? Have any of you tried something like that?