Can two circuits' neutrals be tied together (not a single neutral wire, but two that have been connected)?
This is a safety hazard. An electrical system must remain safe (not hurt a user or overheat and cause a fire) even if there is a failure.

In your situation it is true that the neutrals will not be overloaded under normal conditions. But if one of the neutrals were to get disconnected at or near the breaker panel (loose screw or accidental damage to the wire itself) then the other neutral could end up carrying the current for two circuits and overheat and cause a fire.