There is nothing to stop a shakta also being a shaivite, for Shiva and Shakti are intimately related. In practice, the main difference between shaivism and shaktism is merely the emphasis. While the shaivites lay greater stress on the unmoving, all pervading consciousness of Shiva, the shaktas lay more stress on the dynamic aspect of existence or consciousness called Shakti. Of course, they are really concerned with the same thing, and one can worship both simultaneously. If you prefer to direct your spiritual practices to either one of them, then this automatically implies acceptance and thereby worship of the other. Within the realm of Shiva-Shakti is the entire range of possible human existence. So what is to prevent worship of both?
We can go further: every deity of every religion, all the deities of all tantric sects and in fact any deity that has ever been conceived is no more than one of the multitudinous forms of Shakti (or Shiva). There is nothing in the manifested universe that is not an expression of Shakti. So, in a sense, all religions and sects come under the banner of the shaktas. Of course, the shaktas of India have developed some characteristically far out spiritual practices, but this does not detract from the essential universality of the shaktas. All is Shakti and therefore everything can be regarded and worshipped as Shakti.
We want to end this section by emphasizing that the ancient people of the world were fully aware of the power of spiritual practices orientated towards Shakti. Let us take an example. The ancient Egyptians, like tantrics past and present, knew that the manifested world is the key, the door to the beyond. Instead of calling the power in the world Shakti, they called it Isis. Only the name is different; the basis is the same. It was stated: “I, Isis, am all that has been, all that is and all that shall be; no mortal man (man of low awareness) hath ever unveiled me.”
It is only through spiritual practices that Isis, or Shakti, can be unveiled. It is only by raising one’s level of awareness that one can pierce the veil (maya) that surrounds Isis. Shakti was known by other names in other parts of the world, for the ancient sages knew that she is the means to transcend. She is still the means in the present day.