I couldn't find the source code for handling Amlogic's Multi-DTB format so I wrote my own (thankfully, it's simple enough): github: aml-dtbtools
The code is written in portable C and C++11 so those should compile without dependencies (works on 64-bit x86 and 32-bit ARM).
The dtbTool is modified from the dtbTool source of QCOM which is floating around (it requires few modifications, surprisingly) while the splitter is based from reading the u-boot code.
Hope this helps!