In reply to Xing Chen Lee:
When you ask, how safe, I’m not sure this has an answer. Not as safe as single sports climbing, but probably safer than alpine or ice.
The dangers come from various things
You are moving fast and hence thinking fast and might slip etc.
You are probably placing less gear
The microtrax etc. (the only approach I use unless almost scrambling) might pull its anchor, possibly from being jiggled around a lot
If someone falls, they will be a long way away, hence more stretch but also very difficult to reach. Or communicate with
You probably only have one rope, so evacuation more difficult.
You can not lower the second to a ledge if they are hurt.
You will not be initially be mounting a rescue from a belay
Alongside full blown simul is the possibly more common practice of taking a trax with one then deploying it for the odd pitch. For example to remove those annoying 30m scrambles between one pitch and another when you are not sure you have enough rope left, or drag is becoming an issue (you can yard rope through a trax and it will not fall back down), and when the moves off the lower belay will be easy for the second.