The feature is called bifurcation, Haswell/Broadwell E5 Xeons may support it, depends on the motherboard manufacturer and BIOS, Intel consumer CPUs and Xeons based on them generally don't.
The HP Z440 does support bifurcation with the latest BIOS (IIRC, there's a pair of x8 slots), and can be found quite cheaply on eBay. After that, it's less than $40 to put whatever E5 v4 CPU you want in place of whatever was there. There are eight slots for DDR4, so 256GB is fairly cheap to do. The case does have room for 3 drives with tool-less trays, plus you can use the 5¼" bays. And, it directly supports GPUs (an x16 slot) that need only 2x 6-pin power, and can be modded for the 8-pin if necessary.
