Each card occupies both 6pin on the motherboard. Can the 5870 run with no AUX power? Can each 6pin on the motherboard be split into two, so both cards share the same 6pin (power)?
The simple answer is YES. There is no GPU pair concept in macOS (no SLI / crossfire), but simply multi GPU. OSX can drive multiple GPU at the same time, even mix AMD with Nvidia still OK.
On the software side, it very depends on how the software written. However, in general, multi GPU can only be used for compute, but not graphics. No matter how many GPU installed, only the GPU that connect to the monitor will do the graphic part. Therefore, make sure you connect the monitor to the "correct" GPU.
No, only very little software can use multi GPU properly. And almost none of the normal daily regular usage can utilise that. If you has a RX580, then all you need should be just the RX580, the 5870 should be quite redundant in normal daily use.
No, only very little software can use multi GPU properly. And almost none of the normal daily regular usage can utilise that. If you has a RX580, then all you need should be just the RX580, the 5870 should be quite redundant in normal daily use.