You can use them both at the same time, however (AFAIK anyways) people have only gotten it to work where the SD2Vita was ux0 and the PSVSD was uma0. I can't personally attest to whether this works, I'm merely simplifying instructions I found on a particular subreddit that's not allowed to be linked here. Some people say they can't get it working, others say it works no problem, so I imagine it's just one of those things that either works or doesn't.
The setup is pretty simple, it should be:
1). Download and install SD2Vita plugin as normal. You'll want to set this up first, since it'll be mounted as ux0.
2). Download and install USBMC.vpk. Run the USBMC installer, it'll reboot your system. I don't remember if it'll update the tai config for you, but make sure both the SD2Vita and USBMC plugins are enabled under the kernel section.
3). Make sure you have the latest VitaShell installed. Open it, and then you should be able to mount the PSVSD as uma0.
And that's it. You might need to make sure both SD cards you use are formatted via exfat, but otherwise that's all you should need to do.