Assuming this is a US based business, then you will need to file a US partnership return. If both partners are actively involved in operating the business, then all earnings from the partnership will pass through to the partners according to their agreed upon profit sharing ratio.

The partnership itself will not pay federal income tax, but rather the partners will pay federal income tax on their share of earnings. Those earnings will also be subject to self employment tax if they are a general partner. State level taxes may vary depending on what state your partnership is located in.

A number of considerations come in to play when forming a venture with another business owner. In general:
1. Making sure partnership solution is the appropriate vehicle to operate your business;
2. Ensuring that the profit and loss sharing arrangement between the owners is respected for federal tax purposes;
3. Obtaining tax free treatment on the contribution of the businesses;
4. Selecting the appropriate tax method to account for existing built in gains/losses in the businesses and associated deductions;
5. If you have a long-running business, considering whether the disallowance of amortization on goodwill applies; and
6. Planning for the unanticipated events, such as the death of a partner.
This a hand-full of items to consider and many more could apply depending on your particular situation and business deal. Let me know if you need any assistance, I specialize in these types of transactions.