These are good questions to ask! They can be quite complex, since tax law is different in every country (and I don't know anything about German tax law).

Fortunately, the EA Foundation (a European organization that works in several countries) offers a way to make tax-deductible donations from Germany to many different effective charities around the world. I'm not entirely certain of how their system works, but they have a page with more details here. I'd recommend looking into them, and determining whether the slight complication of donating through the Foundation is worth the money you'll save on taxes. (The first link in my post also includes an email address where you can reach the Foundation.)

As for fees; most credit card fees are almost entirely percentage-based, so I generally wouldn't worry much about splitting up a donation between two charities vs. giving to one charity. Depending on the amount you plan to give, you may want to consider something like a check rather than a credit card, to avoid fees, but if convenience will help you become more likely to give, that's important!