As your chassis support 2 PCIe slot you need to use different set of risers (Riser 2 and 3). You can refer "Table 2. Guidelines for systems supporting two PCIe expansion cards" section of below link for details.

https://www.dell.com/support/manuals/en-us/poweredge-r630/r630_om_pub/expansion-card-installation-guidelines?guid=guid-cffffdb6-b72c-44f0-b2d1-4838b9e8ee8c&lang=en-us 

Thank you for your answer.

As I understand I can use riser #1 0999FX only in a system supporting three PCIe expansion cards. But in our case we have the system supporting two PCIe expansion cards. Can you confirm this, please?

I checked other risors and I can't find any solution for installing 2x Samsung PM1735 pci-e cards because I can't properly fix them on the back.

Yes. You can not use 0999FX on R630 with 2 PCIe slot. You can use below risers on this config

Riser 2 (One of below)

CY3R8  - ASSY,CRD,EXP,RSR,CNTR X16,R630

JR5D2  - ASSY,CRD,EXP,RSR,CNTR X8,R630

Riser 3 (One of below)

8KY74 - ASSY,CRD,EXP,RSR,LF FH,IPT,R630

6R1H1 ASSY,CRD,EXP,RSR,LF FH,GPR,R630

Once you have above risers you can install PM1735 on these 2 slots. Can you share the part number of PM1735 which you are planning to use in R630

Thank you.

I checked these raisors and I don't see any option how to fix pci-e card on the back if I use raisor #3.

Part number for nvme: C2KKH / XTTR5

You can connect PCIe card to the riser. Below link have the details. If your HHHL card does not have PCIe bracket you need to connect that first before connecting to the riser. In your case you will be needing one FH and one LP PCIe bracket.

https://www.dell.com/support/manuals/en-us/poweredge-r630/r630_om_pub/removing-an-expansion-card?guid=guid-6b29d012-c73e-41d2-866b-e88f634f5786&lang=en-us