This is with Ubuntu 18.04 on a Dell 7920. So the Dell power light goes on but I get nothing aside from the occasional flickering of the HD light, trying to boot when I put the 2080 in my box. (I have two actually and same thing with either one.) Everything boots fine when the RTX comes out. The monitor is plugged into an NVIDIA P2000 which came with the system. Same results before and after updating to most recent driver (415), which is explicitly supposed to support the 2080--though the problem happens so early on it seems unlikely to be a driver issue. I'm not even getting the Dell logo.
It's a fresh system, but legacy boot has been enabled and a few other changes have been made to ensure the HD can be encrypted. The computer booted with the 2080 before these changes were made, but there's probably no going back since the disk encryption is mandatory. (And the IT guy started his vacation today.) I've even tried different PCIe lanes, no change.
What can I try?
UPDATE: So on a hunch (anticipated by @SiXandSeven8ths) I took out the P2000 Quadro and things booted just fine, again with the 415.23 NVIDIA drivers. nvidia-smi works and everything appears in order. Is it really impossible to have both cards in the system at the same time (with nvidia drivers)? I'm happy to lose fancy graphics from the Quadro if need be, I just need it to run 2 monitors for mostly coding while the 2080s do ML. This post for example suggests that it is possible (see iamacow's response, 1st para.)--any suggestions how?
UPDATE 2: So it turned out to be surprisingly difficult to get things going even with a new low-end graphics card (GeForce 1050Ti). Two 2080Ti's did not work immediately; booting hung when the OS got to the point where it started the Gnome Display Manager. (Incidentally, for CUDA 10, needed to go with driver 410.48, manually installed from a .run file.) Reinstalling drivers did nothing, but everything 'just worked' when I switched from GDM to LightDM. To get the monitors to display from the new card, I had to get the GPU ID numbers from NVIDIA X Server Settings (different from what they are in nvidia-smi!!) and manually edit xorg.conf accordingly; doing it from the Settings GUI didn't seem to work, at least on the first try. But now, everything appears to be working as intended.
Last note: A fun fact about the 7920 is that despite packing a 1400W PSU it only has 3 PCIe 8-pin drops, while for 2 RTX's you need 4. A splitter solved this and since these cards max at about 280W I'm not too worried about overloading anything, though full disclosure the system has not been fully 'stress-tested' yet.