I used this Z370 motherboard with Intel Core 7 and 32 GB of ram with EVGA Geforce GTX 1070 GPU card very successfully for couple of years. Troubles started when I decided to add another totally identical GPU (same GTX 1070; twin sister). When I boot to Windows the device manager shows h error code 43 for one card and it got disable. This happened with two separate cards, or connected with SLI bridge.  When I leave either card in place (same slot or change them to opposite), every combination card/slot works perfectly OK.  Nothing wrong with the either card; the two-card combination shows in BIOS setup occupying their slots (x8).  The driver info is the same (latest NVIDEA driver).  Disabling and enabling the problem card gives the picture that they both work properly; however there is no output (video or audio) from one of them. Restarting computer returned to the same point.
Any idea what could be a problem?
Thanks in advance!
Sincerely,
Vladimir