Can two people edit the same proposal at once?

No, so no one overwrites another's work. Only one person edits a proposal at a time; everyone else can view it but can't save until it's free.

1. 1If a teammate is already editing it, you'll see a notice when you open it: «X is editing this proposal».
2. 2Meanwhile, the «Save» button is disabled (you can look, but not save).
3. 3The lock releases on its own when that person closes the editor or leaves it idle for a couple of minutes.

Consejo: If you absolutely need in, click «Take control» in the notice (the other person will no longer be able to save).