Why two unicode-characters are not allowed in xkb?

I wonder why a sequence of Unicode-characters is not allowed on one key-press in xkb. eg. I use Indian keyboard for typing in Devanagari. This keyboard is based on the inscript keyboard layout. In this layout, there are 4 characters (let's say complex characters) which are not found in the xkb version (Indian). Those characters are composed of three characters and if one wants to type them, he/she is supposed to type all three of them. The characters and there composition is as follows -

1. त्र = त (U0924) + ् (U094D) + र (U0930)
2. श्र = श (U0936) + ् (U094D) + र (U0930)
3. ज्ञ = ज (U091C) + ् (U094D) + ञ (U091E)
4. क्ष = क (U0915) + ् (U094D) + ष (U0937)

In other operating-systems this layout works fine. Why it does not work in Linux? What has to be done to allow this kind of programming?