Everyone uses Tulsi leaf when they worship Lord Vishnu and many people say that Tulsi was Vishnu's another wife. Is this true?
As mentioned here in our Hindu Mythology that The Tulsi is very commonly worshipped by the followers of Vishnu; and the plant is most carefully tendedas his representative. The origin of the worship of this plant is said to be the following:
A woman named Tulsi engaged in religious austerities for a long period, and asked, as a boon, that she might become the wife of Vishnu. Lakshmi, hearing of this, cursed her, and changed her into the plant which bears her name. Vishnu, however, comforted his follower with the assurance that he would assume the form of the Shālgrāma, and continue near her. * The Vāyu and the Padma Purānas teach that the Tulsi was one of the products of the churning of the ocean.
So Tulsi is consort of Lord Visnu in form of Shālgrāma but before getting cursed to be a sacred tree she got married with Sankhachooda as mentioned here in Devi Bhagwatpurana
Devi Tulasi was the daughter of Dharmadvaja and she too turned out to be a Tapasvini, performing extremely severe sacrifices like meditation with ‘Panchagni’ or Fires on five sides of her body - on four direcions and on top. Lord Brahma knew her desire of wedding Narayana Himself. He explained to her that she was fully aware that She was a ‘Jatismara’ ( person having knowledge of previous birth), that she was Devi Tulsi Gopi in her previous birth as a maid of Radha desirous to have union with Krishna in a ‘Rasakreeda’( dance performance) which was not liked by Radha who cursed her to quit but Krishna advised her to take a fresh birth as the daughter of Dharmadhvaja as a Tapasvini. Brahma in turn explained to Tulsi that there was a Gopa ( Cow boy) named Sudama who was of Krishna’s ‘Amsa’ ( a part Extension) - also a ‘Jatismara’- as he too was cursed by Devi Radha to become a demon now called Sankhachooda. After marrying him, Brahma said that she would eventually attain union with Krishna as the latter would curse Tulsi to become a Sacred Tree to be prayed to all over the world. Brahma blessed Tulasi to become a Siddha to secure a good husband and also taught to her Radha Mantra, Radha Kavacha, Radha Stora and the method of Radha Worship to conquer her confidence and in the long run she would secure Krishna Himself. At the same time, Sankhachooda too got enamoured of Tulsi and, with the blessings of Brahma, married her in Gandharva style.