What you're talking about is a suspended compound or a suspended hyphen. It's a stylistic choice, but it's perfectly acceptable according to the majority of grammarians:
From the Government of Canada's TERMIUM Plus:
When two or more compound adjectives contain the same word, to avoid repetition, writers usually omit that word from the compound(s) at the beginning of the series. Thus, first-class and second-class fares becomes simply first- and second-class fares, with the word class omitted in the first compound adjective.
Note that the hyphen before the omitted word is retained: first- and second-class fares (not first and second-class fares).
This structure is called a “suspended compound.” Here are some more examples:
high- and low-pressure turbine
interest- or revenue-producing schemes
short- and long-term plans
two-, four- and six-metre widths
From The Chicago Manual of Style (17th ed.), 7.88:
When the second part of a hyphenated expression is omitted, the suspended hyphen is retained, followed by a space (or, in a series, by a comma).
fifteen- and twenty-year mortgages
Chicago- or Milwaukee-bound passengers
five-, ten-, and twenty-dollar bills
From Mignon Fogarty's blog post "How to Use a Hyphen":
You can also suspend hyphens. No, it doesn’t mean they got in trouble at grammar school, it means that to save space, you can suspend hyphens when you’re listing several words describing the same noun. How do you suspend them? Let’s say Santa found a fire-proof, dog-proof, and soot-proof vest online. You don’t need to write the full compound adjective each time, since each one is modifying the same noun, “vest.” Instead of writing “proof” each time, you’ll list them, each with only the first part of the compound, followed by a hyphen and then a comma. So if you were suspending hyphens when listing what type of vest Santa was planning on buying, you’d write that he purchased "the fire-, dog-, and soot-proof vest online."
Unless you are following a style guide that specifically gives different guidance, there is nothing wrong with using any of these presentations:
Velocity-proportional forces and displacement-proportional forces.
Velocity-proportional and displacement-proportional forces.
Velocity- and displacement-proportional forces.
You can also choose to rephrase it completely:
Forces that are proportional to velocity and displacement.
Which presentation you use is entirely up to you.