Each twin is traveling in spacetime.
Which means that each twin is traveling both in space and in time.
In your example you have only included 2 of the 4 speeds that take place in the experiment.
Lets name the twins:
Left Twin is A Right Twin is B
A has 2 speeds in spacetime and so does B.
A's speeds are
- Vax = (velocity of A in space)
- Vat = (velocity of A in time)
B's speeds are
- Vbx = (velocity of B in space)
- Vbt = (velocity of B in time)
If we combine those speeds we get:
Vax + Vat = C
Vbx + Vbt = C
Both objects are traveling in spacetime at the speed of C. Which is the speed of light.
So both objects are traveling at the speed of light in spacetime. The reason why theythe twins will think that their clock is running faster or slower than the other twin's is, is because they are both unable to observe the other twin "from above""from above" (like we do in the picture that you provided). They instead, observe theeach other teen while considering their own selvesthemselves as stationary!
But that's "wrong"...* Neither of them is stationary... They are both moving in spacetime at the speed of light. But since we have the equivelence principle, we can infact consider ourselves as stationary and think that they other guy is moving in space or in time, either faster or slower... That creates the "paradox" that you are asking about. Its not a paradox, is just shortsightedness...
(Its not really wrong, we are just unable to see spacetime "from above" like in the picture you provided, so we end up seeing all those effects of relativity, like time dilation and length contraction. When we observe both objects from "above" all those effects vanish).