Yoga is derived from √Yujir(Join) root and Tantra from √Tan(To Expand) root. For Yoga, there need two objects but for Tantra, only one object is required. Through Yoga, the union is achieved, but through Tantra, expansion is achieved. Yoga says That Atman united with Paramatman, whereas Tantra Says that expanded Atman is Paramatman. Tantra and Yoga united in Sanatan Dharma [spirituality]. Purpose of Yoga is to increase and expand the consciousness and purpose of Tantra is to go beyond of consciousness.
Consciousness has intentionality as mentioned by Western philosophers like Husserl and Sartre. Tantra helps to cross the boundary of intentionality. Yoga joins the lower intentionality (human) with higher intentionality (God). Tantra puts beyond the intentionality.