It would sort of have to be, wouldn't it?
If a single-class martial or a single-class caster has a power level of 1 then the martial power plus the caster power of a half-caster should in the name of balance also add up to about 1.
Otherwise, why would anyone play just a martial or just a caster at power level 1 when this gives them power level 1+1?
Not necessarily, or at least, the problem is that you have boiled a multifaceted thing down to only one dimension, which destroys the important details where the differences can add up.
Consider, for example, a Wizard class, designed to break the unfortunate 3E-derived "spellcasters rule, martials drool" pattern. It heavily restricts direct offensive damage, to focus on control, manipulation, buffing, debuffing, area-denial, and diverse utility effects. In this paradigm,
fireball isn't just a really good damaging offensive spell, it's
one of the only damaging offensive spells at 3rd level.
Then we have a Fighter, geared for being great at three things: Physical demonstrations of skill, making things exceptionally dead, and surviving against ever-more-ridiculous threats. That first thing includes skillful use of
equipment, meaning adventuring equipment rather than arms or armor, something I think D&D has unfortunately rather neglected, even my preferred edition.
Under this paradigm, it's quite possible to have a Swordmage that is still "100% magic" and still "100% martial", but it's doing those things differently and in concert. The overall power is still "1" as you put it....but the martial and the magic are things which
overlap and intersect, so that it would be erroneous to count it as "2"--because some of the things that are "magic" are
also things that are "martial".
For instance, perhaps the swordmages' art evolved out of ancient smithing techniques, so it includes some amount of equipment-usage skill....but
specifically through a magical lens, such as "you can bind Runes to your equipment" (again, with "equipment" including non-arms, non-armor items), allowing you to do things with otherwise "mundane" items that others couldn't. Perhaps a rune of Air, applied to a rope, could make a rope that will
rise upward as you uncoil it, allowing you to easily climb surfaces. Perhaps a rune of Fire could be applied while setting up tents, allowing all of the benefits of an actual campfire
without wood, smoke, or light, making for perfect stealth while still eating cooked food.
Then, when we turn to the actual
fighting side of things, runes bound to your weapons or armor could have similar utility in a variety of ways. An Air rune bound to boots could give bursts of speed or even flight. A rune of Earth could be bound to a weapon, allowing it to crush with the force of an avalanche. A rune of Water could be bound to a suit of armor, perhaps cleansing the wearer of negative effects. Etc.
Now, I'm absolutely 100% spitballing here, and an actual design might look completely different from this. But the point is to show how you could have a thing that is a
synthesis of the concept of being a Fighter and the concept of being a Wizard, where most-to-all of the "magic" actions they take (not actual
spellcasting) are one and the same as the "martial" actions they take
and vice-versa. Meaning, as I said above, you
cannot separate it into two bins, the "magic" bin and the "martial" bin--every action is, in some way,
both things at the same time. Then, this "well 1+1 > 1, therefore this is inherently busted" logic cannot hold, because mathematically it
isn't 1+1, it's comparing {A,B,C,D} to {M,N,O,P} to {(A+M)/2,(B+N)/2,(C+O)/2,(D+P)/2}, or whatever. It's not a simple sum anymore, and we aren't working with ordinary arithmetic.