I'm not a member of staff, but you can credit more than one. Community member GrandmaT wrote it all out in a gallery post:

https://www.renderosity.com/mod/gallery/helpful-hints-on-vendor-credits/2642414/

I do find it's still a buggy system (in Firefox anyway). I'll add a few of my credits and then it will refuse to accept more, for whatever reason. I've just taken to adding what I can, which is a bit unfair to vendors, but...

Hope this helps.