How to book several different items / rooms / facilities (several calendars) with one booking form ?

This feature is available only at Booking Calendar Business Medium or higher versions.
You need to create new custom booking form at the Booking > Settings > Form page, and define several calendars.
Example, of configuration booking form with several calendars of different booking resources:

<style type="text/css">
	.flex_container { display: flex; flex-flow: row wrap; justify-content: space-around; }
	.flex_item { flex: 1 0 auto; }
</style>
<div class="flex_container">
	<div class="flex_item">[calendar]</div>
	<div class="flex_item">[calendar id=1]</div>
	<div class="flex_item">[calendar id=2]</div>
</div>
<div class="standard-form">
     <div class="form-hints">
          Dates:[selected_short_timedates_hint]  ([nights_number_hint] - night(s))<br><br>
          Full cost of the booking: [cost_hint] <br>
     </div><hr/>
     <p>First Name (required):<br />[text* name] </p>
     <p>Last Name (required):<br />[text* secondname] </p>
     <p>Email (required):<br />[email* email] </p>
     <p>Adults:  [select visitors "1" "2" "3" "4"]</p>
     <p>[submit class:btn "Send"]</p>
</div>

Where [calendar] - default calendar (you define this booking resource in the Booking Calendar shortcode, during inserting it into a post or page);
[calendar id=2] - calendar of booking resource with ID=2;
[calendar id=3] - calendar of booking resource with ID=3;
etc...