How about creating a group named PRI with dynamic condition such that it populates when Caption contains PRI. Set it for BEST STATUS. Trigger alert when the group is down.
Create alert suppression for this alert. First define a BEST STATUS group (named BCK) where Caption contains BCK. Configure alert suppression such that the alert is suppressed when group BCK is UP.
Make another group named PRIandBCK. Defined as dynamically populated by Caption containing PRI or BCK. Configure the group BEST STATUS.
The 'complex condition' (Tip #3) scenario described here is an option: https://thwack.solarwinds.com/community/solarwinds-community/product-blog/blog/2015/03/12/cutting-down-on-alerting-noise-guest-post-from-support