I mentioned this a while ago, and was told it was because adaptive sampling wasn't being used in multi GPU mode in that version, but would be in the next release. I can say it still is the same, and I thought maybe it was just my hardware, but then I tried it with LuxCore render, and it is actually what I would expect: LuxCore doesn't display the finished render time, but they show how many Samples per sec. and it represents as I would expect, CPU only=2.9m/s, CPU+1GPU= 4.9m/s, and CPU+2GPUs=8.3m/s (more samples per second is faster) It is also consistent if I use the cpu and my slower GPU it's a bit slower than if I use the cpu with the faster gpu instead. Basically the more devices the faster the render time, but with ProRender, it's like this:
500 sample test
RX5700 XT = 30.32 sec.
RX5700 XT+RX580 = 27.92 sec.
RX5700 XT+RX580+CPU = 31.79 sec.
So adding the RX580 to the main GPU only improves speed by a very small margin, and in longer renders that spread gets even less. Then adding the CPU to the both of them actually hurts the speed, and that difference actually gets greater on longer renders.
I don't know why this happens, but I have observed that when LuxCore is doing a final render, on all three devices, it actually uses all threads of the CPU to 100%, and BOTH GPUs are almost 100% use, and it stays that way until it is finished, and the 5700 is using 160-210watts while the 580 is using 110-180watts through out the entire rendering process.
Now with ProRender, with all 3 devices used, the cpu only uses about 30%, and each gpu does go to almost 100%, but only alternating, each GPU for about 3-4seconds, they go back and forth, but never together, and the CPU seems to be doing similarly, but not as much. Whereas the GPUs drop to 0% when the other one goes up to 100%.
I have had other people here say that, at least with 2 matching Nvidia cards, that they saw about 50% cut in a render time with both cards over 1 GPU, I don't know but it would be nice to get the benefits of having my second GPU, because tossing the load back and forth doesn't really seem to be benefiting anything, It sure would be great to be able to get a bit more out of having both GPUs in my system.