How to type zero initial in the common Shuangpin Pinyin keyboard layouts?
The Chinese Wikipedia page https://zh.wikipedia.org/wiki/%E5%8F%8C%E6%8B%BC list three different strategies for handling zero initials in 双拼:


  零声母方案：指全拼中无声母的音节在双拼中的处理方案，即全拼中有几个音节没有声母，双拼中需要把这个零声母实质化，于是就有了零声母方案。
  
  
  固定一个字母做零声母 + 韵母所在键，如微软双拼、智能ABC、紫光双拼
  以韵母首字母为零声母 + 韵母所在键，如拼音加加(注：拼音加加实际上兼容两种零声母方案。)
  单韵母、三韵母为韵母首字母 + 韵母所在键，双韵母为韵母首字母 + 韵母末字母，如自然码、小鹤双拼
  


Which are:


Use a fixed letter + final (Microsoft, 智能 ABC, 紫光)
Use the first letter of the final + final (拼音加加)
Use the original form of two-letter finals. For others, use the first letter of the final + final. (自然码, 小鹤双拼)


For 'ang', 1. would be some fixed letter + the letter for 'ang'. 2) would be 'a' + the letter for 'ang' 3) would be 'a' + the letter for 'ang'

2 and 3 differ when the final contains two letters. For example, 'an' would be 'a' + 'n' in 3, but 'a' + (the letter for 'an') in 2.