How come I see two ICE trains scheduled for the same platform at the same time?

Unless a delay is indicated for one of the trains, it is quite likely that these will actually be the same train for some part of the journey, and get split up at some way point. Check the map for the routes of these two trains, see if they coincide initially - to make sure.

If you can spare the time, go to the Reisecentrum - the DB service center at the train station - and ask. You can skip the line and go to the "reception" counter if they have one.

Also, when the train doors open, go inside and look at the LED display which is (always?) present and will tell you which part of the train you're on. Some trains may have these LED displays outside instead of / in addition to the inside displays, YMMV.

Note: If you're travelling past the point of separation, then at that point you must be on the part continuing to your own destination. On most/all ICE trains with this partition - you can't move from one part to another while on board. You might be able to make the switch at a station before the partition, but don't risk it - make sure you board the right part to begin with.

Usually, those would be multiple units going to different destinations. It's quite common in several European countries, for ICE in Germany, TGV and Thalys in France, regular Intercities in the Netherlands, etc. Both halves of the train will travel together on part of the route and be split at some intermediary stop. Concretely:

• If you don't need/have an assigned seat and your destination is on the first part of the route, you can get in either unit.
• If seat reservation is mandatory, the trains will typically have different numbers and/or a different set of carriage numbers and you need to pay attention to the signage on the train itself (nowadays usually LED but sometimes still sheets of papers attached to the doors' windows).
• And if there are no seat reservation and you need to go to one specific half of the train, you have to figure what the final destination of your half of the train is and orient yourself based on that.

Although it is rare, another possibility is that there are really two distinct trains on that platform. The platform names might be slightly distinct (“1a” and “1b”). Either the trains will leave in different directions or one of them will use a switch to go to a third track in-between the platforms. Personally, I have never seen that with ICE in Germany and, more often than not, there will be a few minutes between the departure times but that's something to keep in mind if you encounter such a confusing situation elsewhere.