Captive portal with data limit and login multiple times

So we have setup our Mikrotik against a Radius server so that users can login. Users have a login and can authenticate and do internet. We can limit a user for like 1GB of data for a time period this works fine. The problem is that a user can only login on one device at a time if they go from for example their Iphone to their laptop the iphone the user is logged out. We can measure the data session or what is remaining during the first login and what is downloaded during that session. However it seems impossible that mikrotik supports 2 or more logins on the same user when there is a data cap active. Anyone any ideas.