R640 RISER CARD A2

If your current riser is Raiser 1A with 2 LP PCIe slot then you can have Riser 2A. Please note riser 2A need both processor to be installed. You can refer below link for details

https://www.dell.com/support/manuals/en-us/poweredge-r640/per640_ism_pub/expansion-card-installation-guidelines?guid=guid-98474365-a5ff-4468-8960-a4f7ac4fb2c5&lang=en-us 

I see riser 1B can be used with processor 1, am I right?

1B and 1A can not be installed on same server. Supported configurations are 1B only or 1B+2B

AS I understand from the configuration mentioned below is that Riser 1A + Riser 1B can be used together for processor 1 which means I can combine them for the same processor 1 which match my configurationhttps://www.dell.com/support/manuals/en-us/poweredge-r640/per640_ism_pub/expansion-card-installation-guidelines?guid=guid-98474365-a5ff-4468-8960-a4f7ac4fb2c5&lang=en-us

Server have 2 riser slot Riser 1 and Riser 2. In riser 1 slot you can connect riser 1A and 1B and in riser 2 slot you can connect riser 2A and 2B. Hence you can not connect both Riser 1A and 1B on same server as there is only one slot for Riser 1. You can check below link for riser connector

https://www.dell.com/support/manuals/en-us/poweredge-r640/per640_ism_pub/system-board-jumpers-and-connectors?guid=guid-b0386e19-b5f6-4cb7-9a6d-13acfcc76058&lang=en-us

You can also see various riser combination supported on the link you provided. If you scroll down table 2 to 5 represent various riser combination supported 

So, is there any suggested way to install my PCIe since I installed one NVMe PCIe and also dell installed Boss M2. on the server

what do you suggest?