Generally, the IRS will assume each co-owner owns a 50% share, but that can be rebutted if you have documents (say, divorce order that the division is 46/54 instead of 50/50). If you file a return where the 1099-S does not match what you report, you might get a letter from the IRS but you can easily respond to that.
And, be aware that you might qualify to exclude some of the gain under the "unforeseen circumstances rule." See publication 523 on page 6.
https://www.irs.gov/pub/irs-pdf/p523.pdf
