How to use multiple client VPN interfaces

I have a rasp pi with OpenWRT, which is connected to the main router via wire, OpenWRT WIFI is disabled for now. The goal is to use rasp pi as the 2nd router behind the main router, for VPN connections only. I have set up two Wireguard interfaces on my rasp pi, both are client connections to remote Wireguard servers. However, I do not know how to properly use them. For now, I disabled one so only the remaining one is active.

How could I control the access for connected devices? It would be nice to allow devices to choose between vpns upon connection, or, at least, hard code devices (IPs?) to use either one vpn or another. How do I do that?

Or, maybe there is a better practice for that, like separate SSID per vpn interface? Or use VPN1 for URL/address ABC and VPN2 for the rest.
Thank you in advance!

You can use Policy Based Routing (PBR) to achieve what you want:

If your raspi supports wifif and you can make multiple SSID like a guest wifi:
https://www.google.com/search?client=firefox-b-d&q=openwrt+guest+wifi

You can route one guestwifi via the VPN1 and the second guestwifi via VPN2 with the help of PBR.

You can also route certain LAN ip address to route to one or the other VPN and switch ip address on the client.
For Windows users you can use a utility called NetSetMan or Free IP Switcher or do it manually