Also, it's likely to be connection of Shiva and Vishnu. So my question is, do any scriptures mentions about it relatively?
The only combination kind of thing i can think of is the story of the Sage Upamanyu and the Lord Sureshwara form of Bhagawan Mahadeva.
Desirous of ascertaining whether I was solely devoted to him and him alone, he appeared before me in the form of Sakra surrounded by all the deities. As the celebrated Sakra, he had a thousand eyes on his person and was armed with the thunderbolt. And he rode on an elephant whose complexion was of the purest white, with eyes red, ears folded, the temporal juice trickling down his cheeks, with trunk contracted, terrible to look at, and endued with four tusks. Indeed, riding on such an elephant, the illustrious chief of the deities seemed to blaze forth with his energy.
Within the twinkling of an eye, however, I saw the celestial elephant I had beheld before me transformed into a bull as white as a swan, or the Jasminum pubescens, or a stalk of the lotus or silver, or the ocean of milk. Of huge body, the hair of its tail was black and the hue of its eyes was tawny like that of honey. Its horns were hard as adamant and had the colour of gold. With their very sharp ends, whose hue was a mild red, the bull seemed to tear the Earth. The animal was adorned all over with ornaments made of the purest gold. Its face and hoofs and nose and ears were exceedingly beautiful and its waist too exceedingly well-formed. Its flanks were possessed of great beauty and its neck was very thick. Its whole form was exceedingly agreeable and beautiful to look at. Its hump shone with great beauty and seemed to occupy the whole of its shoulder-joint. And it looked like the summit of a mountain of snow or like a cliff of white clouds in the sky. Upon the back of that animal I beheld seated the illustrious Mahadeva with his spouse Uma.
As for symbolism, both Bull and Elephant represents Auspiciousness.
Elephant is a symbolises Mata Lakshmi, Lord Ganesha and Lord Indra.
Mata Lakshmi is Sri, Lord Ganesha is Vighnaharta and a symbol of Shubhta, in BVP he is also called as a form if Lord Jrishna and Lord Indra symbolises Swarga and he is also SachiPati. Mata Sachi is also called as Swarga Lakshmi.
Sri, Shubhta and Swarga can mean Auspiciousness.
It's said that the name of the vahan owl of Maa Lakshmi is Ulupi, which is also one of Lord Indra's name.
Bull is the symbol of Bhagawan Shiva. Name of the bull is Sri Nandi.
Nandi means Happiness, Shiva means Auspiciousness.
I hope this helps. Prd..