1- Can you use bandage or sedatives on an other investigator on your space?
What does the card say?
The wording on both these cards suggest that you can only take the action/task for yourself.
If you want another Investigator to make use of them, then you'll need to Trade first.
Action: Discard up to 2 facedown Damage. Then discard this card.
To me, that suggests that you can only apply it to yourself.
Page 6 of RR wrote:
Component actions on cards are preceded by the word “Action” in
bold and can be performed by the investigator who has that card.
At the start of your turn, you may discard 3 Damage and 3 Horror. If you do, discard this card and end your turn.
Again, that suggests only you.
2- when you can heal wound or horror, can you choose which one you discard?
If the card is face-up then you decide which to discard. If it's face-down then it's random.
Page 7 of the RR wrote:
When an effect allows an investigator to discard a Damage or
Horror, he may discard a faceup card of his choice or a random