There has already been a great deal of thought and diplomatic effort put into considering where borders would lie within a two-state solution. Negotiators would not be starting from scratch (as they would under other proposed solutions).
There are logical places to establish borders. The armistice lines drawn at the end of the 1948 Arab-Israeli war (also known as the “Borders of 1967”) were agreed upon by both sides. This could work as a framework for any two-state solution. There would have to be some additional negotiation but individual land swaps could be orchestrated by mutual agreement. For example, a Palestinian government could allow Israeli settlements in the West Bank to be incorporated into Israeli territory in exchange for sections of Israeli territory. Even sticking points like Jerusalem could be solved within a two-state solution. The 1947 Partition Plan, for example, sought to make Jerusalem an international city. This would grant both Muslims and Jews unrestricted access to the city, without forcing the two to live under a single state.
The borders established under the terms of the 1948 armistice were not effective because they could not adequately solve the issue of the West Bank. If those borders were adopted, hundreds of thousands of Israelis living in the West Bank would be forced to live in a Palestinian state or forcibly removed and relocated to Israel. This would be unacceptable. Many of the settlements in the West Bank are already fully established cities. Ariel, for example, has a thriving economy and a university. Relocating these cities would be impossible.  The only other option would be to make these settlements part of the Israeli state in a mutually agreed swap. A Palestinian government would be unlikely to agree to this as it would deny them a contiguous territory, forcing citizens to pass border checkpoints and enter Israeli territory when passing between Palestinian cities. Jerusalem also represents a major sticking point for a two-state solution. Temple Mount, the site of the Western Wall and the al-Aqsa Mosque, is a holy site to Muslims and Jews. Dividing it up and excluding Jews or Arabs from certain holy sites would be unacceptable to both sides. The only way to maintain shared access to Jerusalem and its holy sites would be if both Jews and Arabs lived under a single state. This would allow both religious denominations to visit the holy sites unimpeded and obstructed by border crossings.
[P1] There are borders which have already received Israeli and Palestinian agreement. [P2] Therefore, a two-state solution makes sense as negotiations would not have to begin from scratch.
Rejecting the premises
[Rejecting P1] These borders were agreed before further Israeli settlement expansion in the West Bank. There are now hundreds of thousands of Israeli Jews living outside of the 1967 borders.