Multiple one to many relationships between tables
If a task can have multiple assignees, then you may want a many-to-many table between employee and tasks.  However, the current model works well if the task can only be assigned to one employee at a time and you don't want to record assignment history.

It is not uncommon to have multiple one-to-many relationships between two tables.  You have correctly modeled the case you have described.   However, I generally separate created-by and owners relations.  This permits someone other than the owner to enter proposals and tasks.  A created-by fields is typically used as an audit field which should not be modified, while the owner field usually would be modifiable.

You do have a many-to-many relationship between employees and proposals. This resolved by the tasks table.