Depends on your gear. If you can efficiently get your magic tank to full resistance while still pumping up their DEF, then they can work as a solo tank if the boss uses AoE magic + ST physical.
Same is true of physical tanks if a boss uses AoE physical and ST magic attacks.
If you have to cover non-elemental magic damage, you will probably want 2 tanks, so you can stack SPR/HP on the magic tank , and let a separate provoke tank draw all the normal attacks.
All depends on exactly which attacks you are dealing with. I often just bring Mercenary ramza as a solo cover/Provoke tank. His resists are fantastic, so he can usually get full immunity to certain elements without losing too much physical eHP. And despite the "downside" of his magic cover being a cooldown, bosses that dispel are so infrequent, it is not much of a bother.