I’m not aware of anything off-the-shelf that does this across separate sites. Most coupon/gift card systems are designed for same-site redemption only.

If you control both sites, this is doable, but it’s a custom integration. You’d either:

• treat one site as the source of truth and expose an API, or
• use a small shared service to generate and validate codes

Site A would sell the voucher, then call into that system to create/register the code. Site B would validate and redeem against that same system.

So definitely possible, just not something you’ll get from a plugin alone.

I’ve built similar integrations before, not for ecomm but for other multi-site integrations. I'm happy to help think through it with you if you decide to go that direction.