Hey there @TSHY - good question! =)
For solo users, this is how Calendly is designed to work. Because account is seen as a single person that cannot be in two places at once, Calendly will not allow double-bookings. This means that when one event type is booked, other event types will automatically be blocked off at that time. There is no way to change this.
However! You can work around it by setting up a team page that includes multiple users that are actually your alternative email addresses. Then, create an even type for each user under the team page - making sure each event type has a different host - so they can all be booked simultaneously. (Note: one-on-one events cannot be created on a team page, but you can use a Round Robin Event Types as a one-on-one, and read more about how to do so here!) Keep in mind, you will have to pay for each additional user.
I hope this helps!