MikroTik

I'm using Radius to assign Rate-Limit to Hotspot users.

Is it possible to group two users together to share a Rate-Limit?

For example - two different MACs that share a 512k rate-limit. I read about Profile based Rate-Limit when using PPP, but I am not using PPP.

Which radius server?

Freeradius

I'm not sure how to do it in FreeRadius, but in Radiator it's pretty easy to set multiple profiles and have each client match one or the default depending on which profile attribute is stored for a given user. There's got to be a similar way to do this in FreeRadius.

I think you misunderstand what I'm saying

I want user A and user B to share a 512k rate limit

So if user A is downloading at full speed, then user B will only have half the available bandwidth.

I do not want user A to have 512k and user B to have 512k. They should share the bandwidth.

You'll have to do this by manually setting up simple queues and marking traffic from both users.
You possibly could do the marking part automatically withthe 'Filter-Id' radius attribute then set a filter rule for each group of users.
Hope I'm making sense!

Yes thank you