Actually, it is level 22 to 24. A Fighter advances 2 levels to a Knight of the Rose's 1. 250k vs 500k. But, hey, believe what you will.
Since you didn't even bother to look it up before you kept digging:
A Knight of the Sword reaches level 18 at 3,850k xp. The fighter is at level 23 at that point.
Sword can't advance past 18, and you don’t want to lose levels by switching orders early, because the only advantage Rose has over Sword in the first place is the ability to level past 18. It takes 5,500k xp to stay L18 when switching, and 6,000k to reach level 19. At that point, the fighter ist about to hit L32, at 6,000k +1.
In summary: I can see how you earned your title.
And in DQK, if you are not clearing entire screens of dragons with DBF, it doesn't matter how many HP you have as they come in groups of a dozen or more in any of the serious fights. Your Dragonlance wielder is useful but aren't exactly going to eat all those breaths and be in any shape left to fight. You only really need about 150hp, which a maxed out Knight will have at level 14.
I play with multi-class characters all the time. You don't need that many HP to win the game. Just enough to beat your typical dragon's HP for a Dragonlance wielder. Which doesn't matter all that much if you plan ahead and have 4 DBF slinging mages in the party, except for that one fight.
You don’t
need that many hp, you don’t
need that many mages either. Two dragonlances and haste goes a long way. As I said at the start, it's nice to have the extra hp cushion in case things go south, but it's hardly essential.
The biggest advantage of Fighter vs Knight is faster leveling. The Knight starts with more hp, and gets Solamnic Plate that stacks with rings of protection; the fighter gets better THAC0, saving throws and #attacks earlier, and has more movement in magical armor. Since the first two games have level caps rather than xp caps, the Knight will catch up in most regards, but it's still noticeable and makes the classes distinct in the early game.
Planning for two dragonlance wielders is just good party building. Bringing at least one Knight is a given. You have plenty of options fore the second one, and single-class fighter definitely is a valid one.