Funny you ask, because I am considering this as well.
Iray will use multiple cards (as well other similar renderers), but as with SLI, the VRAM does not add up. So two cards with 11 GB still count as 11 GB, but obviously with double the processing power.
Now 24 GB sounds like a lot, but while testing Iray in CC, my 8 GB card ran out of juice with moderate scenes. It helps to optimize the scene. Even what is not visible, still has an impact it seems.
Considering the current prices of 2080 Ti cards or even new 1080 Ti cards, of which you would want at least two, one Titan RTX suddenly becomes attractive, and you have more than double the VRAM.
I've started specking out a system for this card, which would be something like this:
Intel Core i9-9900K Coffee Lake 3.6 GHz Processor
Gigabyte Z390 Aorus Pro WiFi LGA 1151 ATX Intel Motherboard
64 GB RAM (maximum that the motherboard can handle unless we have 32 GB RAM modules)
M2 format SSD (2 @ 1TB)
additional drives as needed
You need to be able to handle a lot of data fast, so the processor is quite beefy (8-core) without being too expensive. I always use Intel, but there may be suitable AMD processors as well.