Just because we multi-box doesn't mean we fuse our characters togetehr and they can't be played solo. I for one play from multiple locations, and only one location can support 5 boxing. Sometimes the collection quests I skip while multi-boxing I come back and solo if I have free time at the office. It's a legitimate question that he has about his priest.

I went holy/disc but found soloing to be somewhat of a nightmare when it was my priests turn to do some questing. I tried a Disc/holy build with emphasis on holy dmg, and found that somewhat bareable, however I missed out on some healing goodies. For one particularly long solo questing day, I just respec'd to shadow and when I got back home to my 5boxing rig I switched back to holy.

Tbh though, you might just wanna stay shadow if you're in the low level range. You can heal just fine in instances. If you find it hard to keep your team up as shadow, switch to a discipline / holy build. I wouldn't go pure Holy/disc until I was in my late 60's.