If your RAID is Thunderbolt-based, doesn't it have a Thunderbolt-through port? If so then you should be able to chain a second display to that with the appropriate adapter cable.
Otherwise, Maybe this: https://plugable.com/products/tbt3-hdmi2x-83/ - Not a personal endorsement - just something I've seen advertised, and they explicitly say it works on 2017 iMac (which is important).
Some Thunderbolt 3 hubs will also do it, if you want extra USB ports and stuff, but most of those do it by having a single DisplayPort or HDMI output for one display while supporting a second display on the Thunderbolt-through socket - so you'll still need some adapters.
Pretty sure that youcan'tdo it with non-Thunderbolt USB-C adapters as these rely on DisplayPort's "multi stream" mode to drive 2 monitors on one port, which Mac doesn't support - but a "true" Thinderbolt 3 device can add two full-blown DisplayPort outputs (HDMI on USB-C/TB3 is almost always done internally via displayport) so whatever device you buy triple check that on iMac 2017.