I would buy the PG279UQ, or the Acer X27 (which goes for a few hundred less currently). Its a monitor you will keep for quite a while through several upgrade/builds most likely. I don't see mainstream gaming displays moving past 4K for a while.
I've always been an advocate of getting the single fastest GPU you can afford. I never through dual GPUs was worth it, and its even less worth it nowadays. Scaling is pretty poor in most games. You won't be getting near what 2x the performance of a 2080Ti would be.
Get the monitor, which your single 2080 Ti will do a good job of driving (speaking from experience as I currently have one and a X27). Then your monitor will be squared away and you can just focus on upgrading your GPU in subsequent generations. Just my two cents.