1 external hard drive - 2 macs

I have 1 external hard drive (EHD) (1 TB) and want to backup 2 macs (macbook pro OS 10.8.5 and mac mini OS 10.8.5) using time machine, without time capsule. The EHD is connected with the mac mini, so I want to backup my macbook via wifi. I have set up to share files, but it seems that the backup is being performed on the mac mini instead of EHD (because when I open time machine preferences on macbook, I see that the limit of the backup is the storage memory of the mac mini and not the EHD). Does anyone know what might be happening? I do not want to backup on my mac mini (don't know if it takes up space on mac mini) Want to backup both on EHD. Thanks.