Phanteks enthoo evolv matx tempered glass sli/crossfire

As of this date, there are only six B350 mATX AM4 motherboards that supports 2-Way CrossFire (i.e., x16/x4 speeds minimum). These are:

ASRock AB350M Pro4 (http://www.asrock.com/MB/AMD/AB350M%20Pro4/index.asp)
Gigabyte GA-AB350M-D3H (http://www.gigabyte.us/Motherboard/GA-AB350M-D3H-rev-10#kf)
Gigabyte GA-AB350M-Gaming 3 (http://www.gigabyte.us/Motherboard/GA-AB350M-Gaming-3-rev-10#kf)
Gigabyte GA-AB350M-HD3 (http://www.gigabyte.us/Motherboard/GA-AB350M-HD3-rev-10#kf)
Msi B350M Mortar (https://www.msi.com/Motherboard/B350M-MORTAR.html#productFeature-section)
Msi B350M Mortar Arctic (https://www.msi.com/Motherboard/B350M-MORTAR-ARCTIC.html#productFeature-section)

Since you will be using the Phanteks Enthoo Evolv mATX TG case (it only has 4x expansion slots: http://www.phanteks.com/Enthoo-Evolv-mATX-TemperedGlass.html), you need to select the B350 mATX AM4 motherboard that has its 2x PCIe x16 slots with only a 2x-expansion-slot-distance away from each other (this usually means the 2x PCIe x16 slots must not have any other slots between them) which can accommodate a 2-slot GPU each PCIe slot to cover all 4x expansion slots of the case. The layout should be like this:

[case's expansion slot #1] ===> [GPU #1] ===> [motherboard's PCIe x16 slot #1]
[case's expansion slot #2] ===> [GPU #1]
[case's expansion slot #3] ===> [GPU #2] ===> [motherboard's PCIe x16 slot #2]
[case's expansion slot #4] ===> [GPU #2]

The listed motherboards above will have the following layout when installed in the said case:

ASRock AB350M Pro4
[case's expansion slot #1] ===> [motherboard's PCIe x1 slot]
[case's expansion slot #2] ===> [motherboard's PCIe x16 slot #1]
[case's expansion slot #3]
[case's expansion slot #4] ===> [motherboard's PCIe x16 slot #2]

Gigabyte GA-AB350M-D3H
[case's expansion slot #1] ===> [motherboard's PCIe x16 slot #1]
[case's expansion slot #2]
[case's expansion slot #3] ===> [motherboard's PCI slot]
[case's expansion slot #4] ===> [motherboard's PCIe x16 slot #2]

Gigabyte GA-AB350M-Gaming 3
[case's expansion slot #1] ===> [motherboard's PCIe x16 slot #1]
[case's expansion slot #2]
[case's expansion slot #3] ===> [motherboard's PCIe x1 slot]
[case's expansion slot #4] ===> [motherboard's PCIe x16 slot #2]

Gigabyte GA-AB350M-HD3
[case's expansion slot #1] ===> [motherboard's PCIe x16 slot #1]
[case's expansion slot #2]
[case's expansion slot #3] ===> [motherboard's PCIe x1 slot]
[case's expansion slot #4] ===> [motherboard's PCIe x16 slot #2]

Msi B350M Mortar
[case's expansion slot #1] ===> [motherboard's PCIe x16 slot #1]
[case's expansion slot #2] ===> [motherboard's PCIe x1 slot]
[case's expansion slot #3] ===> [motherboard's PCIe x1 slot]
[case's expansion slot #4] ===> [motherboard's PCIe x16 slot #2]

Msi B350M Mortar Arctic
[case's expansion slot #1] ===> [motherboard's PCIe x16 slot #1]
[case's expansion slot #2] ===> [motherboard's PCIe x1 slot]
[case's expansion slot #3] ===> [motherboard's PCIe x1 slot]
[case's expansion slot #4] ===> [motherboard's PCIe x16 slot #2]

Sadly, all of those listed B350 mATX motherboards will not have enough clearance for the 2nd AMD GPU you will use as the 2nd PCIe x16 slot already fills up the last expansion slot of the case.

So, your only options are:

Option 1) Change the case by looking at an mATX case with at least 5x expansion slots. This will allow the use of your two Sapphire RX580 Nitro+ GPUs on any of the B350 mATX motherboards listed above.

Option 2) Use single-slot AMD GPUs. However, I can only think of the XFX RX 560 4GB though: http://www.xfxforce.com/en-us/products/amd-radeon-rx-500-series/rx-560-4gb-ss-rx-560p4tfgr (I am not sure if there are RX 580 single-slot GPUs). This will let you use the Phanteks Evolv mATX case with any of the B350 mATX motherboards listed above (at the expense of lower graphics power, from RX 580 to RX560).

Option 3) Go with an ATX build (ATX case + either mATX mobo or ATX mobo). This will give you more flexibility and options in choosing whatever motherboard and whatever 2-Way GPUs in whatever case you pick.