4 sentences

1. 第见其一，不见其二。

   dì jiàn qí​yī, bù jiàn qí'èr.

   One sees only one aspect while ignoring the other.

   Source: Dong Chinese Dictionary

2. 他有两子，其一在京，其二在沪。

   tā yǒu liǎng zǐ, qí​yī zài Jīng, qí'èr zài Hù.

   He has two sons; one is in Beijing, and the other is in Shanghai.

   Source: Dong Chinese Dictionary

3. 理由有二：其一，时间紧迫；其二，人手不足。

   lǐ​yóu yǒu èr: qí​yī, shí​jiān jǐn​pò; qí'èr, rén​shǒu bù​zú.

   There are two reasons: first, time is tight; second, there is a lack of manpower.

   Source: Dong Chinese Dictionary

4. 学习汉语分为两步，其一是多听，其二则是多说。

   xué​xí Hàn​yǔ fēn​wéi liǎng bù, qí​yī shì duō tīng, qí'èr zé shì duō shuō.

   Learning Chinese consists of two steps: first, to listen more, and second, to speak more.

   Source: Dong Chinese Dictionary