Subscribing to a general topic and presence and on the same topic name

I noticed in LiveBeats that ProfileLive subscribes to both the profile topic (line 119) and presence separately (line 121). The profile subscription topic is "profile:#{user_id}" whereas the presence topic is "proxy:profile:#{user_id}". In my own toy chat application, I subscribe to both “room” updates and presence updates on the same topic and it seems to work. Should I be making them separate? Are there any technical disadvantages to keeping them on the same topic other than being able to differentiate exactly where it came from, ie, am I introducing any subtle bugs or performance/memory issues or anything?