http://m.focustaiwan.tw/news/aipl/201905200021.aspx
Chen said that under the new legislation legalizing same-sex marriage, as well as the spirit of Judicial Yuan Interpretation No. 748, same-sex couples where one party is a foreign national from a country where same-sex marriage is legal can also register to be married, starting May 24.
Under the new law, titled Enforcement Act of Judicial Yuan Interpretation No. 748, however, even with the legalization of same-sex marriage in Taiwan, a Taiwanese national cannot enter into a legally recognized union with a partner from a country where gay marriage is not legal.
Appears that law only covers at least one person among the 2 who is Taiwanese citizen, and if the other foreigner’s country has the same applicable law. From quick reading, not 2 foreigners.