Short answer, no, there is no way of guaranteeing that another plugin is loaded and remains loaded. The user can unload at any time.
Long answer, if by agreement with the relevant plugin maintainer the function is moved to a common library that can be shared by both plugins it can happen since whichever is loaded first will load the library and it will stay loaded until nobody uses it. There is the utils library that works like this and maybe the function might be added to, but who maintains it should be agreed (and written down :-)
Of course that won't work if the function is not standalone but uses lots more of the plugin's functions.
PS this should be a discussion on G-P but discussions are not enabled on that repo, any reason @frlan ?