There you have it in one 
If HomeAssistant is talking directly to the USB device, then you can’t tap into that connection for anything else.
An option is to remove / disable HomeAssistant from locking onto the /dev/ttyNNNN port and to load a TCP Server application on your PI
(Like VelServ Topic or Velbus-TCP Topic)
And get home assistant to make an internal / loopback connection to that Server app.
When you have that running, talking to the TCP server from anything else (at the same time) is very easy.
I’m not a homeAssistant user, but what I can read is that your YAML file needs to be edited so that the Velbus connection becomes something like
velbus:
port: 127.0.0.1:6000
(Depending on which TCP server you use and how you set it up)
I have some installation scripts for VelServ and Velbus-TCP you can download and use if you want.