Okay, so, when he does the first slash, the momentum from that attack would make it more reasonable to carry through into the uppercut using the green arm, since he's already twisting into a motion where using that arm would use the previous momentum. As opposed to, as you have now, using the red arm for both attacks, having to "reload," so to say, before attacking again. This is why people fight using alternating fists.
I got the impression that he was using a knife, which would preclude switching hands unless you want to throw an unarmed elbow-bash in between the stabbity bits.

However, I'm not sure the two attacks you've got would form a reasonable combo in any case. In order for the attacks to seem like a "combo," they need to flow naturally into one another: your knife/fist should be drawing a long, continuous curve, rather than a series of separate and unrelated lines. Suddenly switching the direction of motion is unnatural (not to mention really hard), and the long pause you put between the attacks shows that you sense that fact, I think. Perhaps try putting the third attack
between these two, rather than after?