The usual RULE states that in every picture, there should be only ONE subject, or else the eye will turn to different directions of the picture frame, and the mind gets confused about the message transmitted by the image.
However, the REAL point of this rule is: to get the message transmitted.
So that means: no matter of the number of subjects, if the message is clear, all is good.
Now, pay attention to the following fact:
If, for example, you have several tulips in a picture, then that means you have only ONE subject – the tulips, even if the number of flowers is grater than one. However, a picture of a garden with several types of flowers AND trees, that is photo with multiple subjects with a clear message “the garden”.
Another note: If you have more people in your photography, that is also not considered a combination of subjects. It is really hard to obtain a combination of subjects using people. But, not impossible – for example, if all the people in the picture composition are doing something totally different, the message would simply be “activities”.
Multiple subjects means that you have two or more objects that don’t relate to each-other other than by forming a symbiotic composition to express a single message.
This type of photographs will always look agglomerated, so do not expect to make a “clean” shot with multiple subjects.
Let’s see some examples of combining subjects in photography:
Outdoor: Winter+Birds => Message (Title): Winter’s wings
Idoor: Cozy oriental reading place + lilac flowers => Message (Title): Lily’s room at dawn.
Macro: Autumn still life + old postcard => Message (Title): Antique