The Ready action is described on PHB, p. 193 or here in the Basic Rules.
You may set a perceivable trigger. The rules are somewhat vague in two aspects:
One might assume, that this means a single trigger, not a bunch of triggers.
It also has to be perceivable, so if you wait for someone to come out of cover somewhere in front of you, you cannot possibly perceive an enemy behind you by common sense. Although the rules do not explicitly say so.
If I had to interpret the rules strictly by the rules and English grammar, there would only be one trigger allowed. The trigger. You would have to pick one of your two.
The way my group plays, we allow multiple triggers if all are perceivable with the same senses and in the same direction. So if you wait for someone to step out of cover and someone in your line of sight attacks you, your ready-action would be valid. But you should consult with your group if they think it's a good way to read this rule.