This one's a "yes, but". Yes, you can do it. But you only get one of those per cleric, and if you've taken a cleric to epic levels, why isn't that cleric using the holy symbol? Even in ToB, that much experience requires that the character spend considerable time with the party.
One technical detail to note: the Shar holy symbol has a "Viconia only" restriction. This is not a problem; while UAI won't help you with most NPC-specific items, it will nullify the restrictions on Jaheira's Harper Pin, Viconia's Holy Symbol of Shar, and Aerie's Holy Symbol of Baervan Wildwanderer.
Oh, and yes, you will reach that level before ToB. Even in my first runs, before I started exploiting the XP system to really push my parties ahead, the party was at more than 4 million XP entering ToB. Really, the worst issues with what you're planning are that Sarevok doesn't fill the same party role(s) that Viconia does, and that Sarevok will be a couple million XP behind the rest of the party.
I'll have Aerie and Jaheira for my cleric needs, so if anything Viconia is just gonna be overkill - but still plan to level her up just for the Holy Symbol. I'll switch npcs around so only charname and Jaheira are going to be super high level come TOB.
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Oh, and yes, you will reach that level before ToB. Even in my first runs, before I started exploiting the XP system to really push my parties ahead, the party was at more than 4 million XP entering ToB. Really, the worst issues with what you're planning are that Sarevok doesn't fill the same party role(s) that Viconia does, and that Sarevok will be a couple million XP behind the rest of the party.