This couldn't be further from the truth. Before the merge there was lots of work being duplicated to different standards. Arcade games, home computers, consoles, etc. share alot of components which were being emulated separately in each project. Bringing the projects together improves the emulation as there are more test cases for the emulated components. Software for home computers in particular pushed the hardware in ways that arcade machines never did and so improves emulation accuracy.
As for contributors, you'll find that there's those that specialise in arcade machines and those that prefer emulating computers, etc. They work on emulating what interests them, and what they have knowledge of. The number of contributors has increased immensely since the merging of projects, and has improved both sides of the project.
Personally, I work on emulating 8 bit computers, mainly Acorn Computers but also anything else that interests me. With the help of the community here I recently improved the Aquarius emulation, and added support for the Aquaricart. I have zero interest in working on any arcade machines.
To improve Intellivision emulation needs someone to contribute that knows the hardware, and understands what certain software is doing to cause emulation issues, and then submit a fix. I could be tempted to add support for more cartridge types if the community makes ROMs available and can provide documentation explaining how they're mapped. Anyone can add newly dumped software to the software list XML and make a Pull Request to the project, ROMs would need to be submitted too to verify the additions.