We can add up to 5 conditions within 1 Event Rule in the Threshold monitoring profiles per documentation.
Why can we only have one Severity per Event Rule when the number of conditions can be multiple.
Release : 3.7
Component : IM Reporting / Admin / Configuration
If you need different severities for different conditions, you would use different rules with the conditions needed to produce the event you wish and apply the appropriate severity to that rule.
Multiple conditions provide the flexibility to have multiple conditions needed to be violated to trigger the event and are not meant to be a substitute for multiple rules.