Just because they are separate application does not mean, that they do not operate on the same data. However I appreciate when I can use the full-blown GUI to look at calendars in another window, even when I'm in the middle of composing an email. The modal switcher in Outlook was one the worst thing I've experienced in productivity software.
For some reason I thought that you where still talking about German provincial government. Which was extremely funny for me to imagine since those guys are not even allowed to speak English in official capacity.
It's not possible anymore because Asian food and chopsticks are a decent and cheaper alternative.
Also not allowed to pack lunch. Also you get penalized for not being able to eat fast enough to meet your targets. Also you have to eat that food 8 hours a day, every day, for as long as you stick around. Well... imperfect analogy but I hope the point is clear. :)
So it's not just the fact that enterprise support for Linux solutions is flaky at best, that the tools themselves are many times nowhere near as powerful as they need to be, or that there is no coordinated strategy to push Linux in the enterprise where it should be going because the promoters never seem to be interested in having the "end-user" tell them what they want and how they want it. Sometimes it can be a lot simpler: the users don't want it. They don't feel comfortable with it so they won't do a good job with it.
This is one of the most common things IT departments all over the world get wrong: instead of enabling their users to do their job better they try to just dictate how to do it with solutions picked in a vacuum.
Just pick the right tool for the job and make sure you have the people using the tool to confirm it's right and you're good.
So your job would be finding those of the few type Y folks and work with them. Even if they are enthusiasts, they still might not see the forest for the trees. Also, that doesn't necessarily mean, that their info you will get you near your objective; you will still have to satisfy many more people, who will not use it, but have their own ideas how it should look and behave, even if it will make life harder to those using it (see also: exactly which programmer wanted open space office?).
There is no perfect answer for these problems.
IT taking decisions in a vacuum without implicating the people who know the business is how you end up spending millions to implement some solution only to see how things get even slower and people spend more time avoiding it, adapting it to their needs, complaining about it, than doing actual work.
Good IT is based on a set of principles that tend to be more stable, not on a set of tools that can change from one day to the next. And when you overlay the needs of your business over those principles you'll come up with the best tool to use. Putting the tools first means you're doing it wrong.
So if it's an accounting firm you might want to talk to some accountants or to someone who really understands their work and their needs before deciding that "EZ Spreadsheets" is the way to go because it's free, or because it's open source (and you love open source), or you worked with it in the past and liked it.
The latter is especially important when you have different conferencing / dial in systems in different regions that all need to be reserved and tied to the same meeting request.
This isn't about having different apps that model has no advantage or a disadvantage the ability to see the presence of people and resources across the entire organization is the big important feature that is not available on other platforms at all or without a momentous amount of work.