Is it possible to connect a few devices to one namespace

is it posiible to connect few devices to one namespace, will it improve IO performace?

Yes & Yes.

namespace <namespace-name> {
  memory-size <SIZE>G  
  storage-engine device {   # Configure the storage-engine to use persistence
    device /dev/<device>    # raw device. Maximum size is 2 TiB
    # device /dev/<device2> # (optional) another raw device.
    write-block-size 128K   # adjust block size to make it efficient for SSDs.