Both Post and Fitzgerald are good at drawing clear silhouettes. The difference?
Post's 2 characters are on one flat plane lined up right behind each other - which is not a criticism, just an observation.
In Fitzgerald's sillo, each character is
1) in a different pose,
2) is positioned at a slightly different distance from our viewpoint and
3) each inhabits all 3 dimensions.
4) Parts of them come towards camera, parts recede,
5) parts radiate all around the bodies. That's extra impressive to pull off in just silhouettes.
Owen Fitzgerald was a multi-dimensional cartoonist. He also had a great style-but the style was merely the final polish on top of a lot of knowledge and skill.
Roberto asked in a comment if it's ok to cheat because most good artists do cheat.
Yes it's ok to cheat - knowingly. On Purpose.
But there is no substitute for knowledge and skill. The more you know, and the more deft your hand-eye-brain control is, the more control you have over what you want to say with your drawings. How many times have you had an exciting idea in your head and then ran to your drawing board to draw it - but gave up after a half hour of struggling because you couldn't draw the thing the way you imagined it? How frustrating is that?
If you can only draw a head from one position, or just one position of a flat hand - that really limits what you can do.
The younger you are, the more important it is to learn as many principles and skills as you can. You can develop style later, after your ability to learn new things slows down.