Is it bad practice to ask more than $1$ question in a MO post, like here?
It can be bad practice. It can also be good practice.
In the proffered example, the questions are brief enough and related enough that I have no strong objection to it. If you have a larger theme (maps from A with property P), and have a purpose in asking multiple questions with different ranges (knowing how it looks on B says one thing, while knowing how it looks on C tells another), it might be good to add a link to the 'mother question' which states the theme clearly. If you are in search of a mother theme, that (along with guesses as to what such a theme looks like) could serve as motivation.
If you have a larger goal (larger than just getting answers to the two questions), I can only see adding it as an improvement.
Gerhard "Tell Us What You're Thinking" Paseman, 2017.11.03.