I thought Vishnu takes an avatar to remove adharma from our planet yet two avatars of Vishnu can exist at the same time on earth? Why is there a need for two avatars at the same time? Parashu Rama and Krishna existed in the same time frame right or am I wrong? Can someone please elaborate?
Yes, Lord Vishnu's multiple avatars can exist at the same time on earth.
Lord Parashurama and Lord Rama were present at the same time (~Valmiki Ramayana: Balakanda: Sarga74-76)
Lord Parashurama and Lord Krishna were present at the same time (~Mahabharata: Udyoga Parva: Bhagwat Yana Parva: Section XCVI)
Lord Krishna and Lord Vyasa were present at the same time (~Mahabharata: Sauptika Parva: Section XIV)
Apart from these instances, Śrīmad-Bhāgavatam (Bhāgavata Purāṇa) consider both Lord Balarāma and Lord Krishna as incarnations of Lord Vishnu (From 4 forms of Lord Vishnu: Vāsudeva, Saṅkarṣaṇa, Aniruddha and Pradyumna; Lord Śrī Kṛṣṇa is Vāsudeva, and Baladeva is Saṅkarṣaṇa). And it's well known that Lord Balarāma and Lord Krishna performed many activities together since their childhood.