Your request probly has to do with licensing and its just much easier, and better to put license checks in the dll as well. There are ways to make it more difficult to use a different injector, but its all not to hard to find out.
A very easy example would be to let your injector write something to the dll memory
in ram, and if the dll doesnt find it, it quits. If you know how to code you can
produce a lot of different methods that make it harder to use a different injector.
More advanced methods could also be to fuck with the NT Header and use manual injection. For example Encrypt important parts of the NT Header of your dll and let your loader decrypt it and inject it.
Also Communication methods like named pipes could be used between injector and dll
and and and.....
edit: i fell into a grave