This is possible. Plus, keep in mind that each spellcasting profession can do things apart from their Bloodstone's specialty. So instead of actually combining the effects of each Bloodstone, the Searing (for example) could have combined the destructive elements of Aggression, Denial and Preservation to create a destructive power greater than possible through the Destruction bloodstone alone.
Alternatively, each of the contributing professions might have their own contribution to the final effect. To use the Searing again, the Elementalist may have taken the leading role, but the Necromancer could have been feeding him energy, the Monk preventing either from bein killed by the energies being generated, while the Mesmer prevents any forces that might normally prevent something like the Searing from happening from interfering.