Don't Say This
"Men piss on their two feet."
This sounds like someone is urinating on their own feet. Unless that's what you're trying to say, I wouldn't use this construction at all.
Simple and Complex Constructions
If you're just trying to keep it short and colloquial, it's probably fine to say:
"Men pee standing up."
However, "piss" and (to a lesser extent) "pee" are somewhat vulgar in American English. More polite phrases might be:
- Men often urinate while standing.
- Men often stand while urinating.
Since men can urinate while sitting, you might also provide more context to explain the point of your sentence. For example:
Urinals allow men to pee standing up without the hassle of raising the toilet seat.
Or you may be trying to draw a contrast between men and women, or men's and women's restroom facilities. For example:
Men typically urinate standing up, which is why men's restrooms usually have urinals installed. Women typically urinate while sitting down, which is why women's restrooms have stalls but no urinals.
Ultimately, the choice of phrasing depends a great deal on your intent and your audience. Context matters!