One of the fun things about percussion is that you can hit just about anything with anything. There are some unsafe ideas out there (brass mallets on almost anything), but hey, live your life. Some pieces, however, get a little too ambitious and make impossible to requests. This primarily comes in the form of changing from one beater to another in a short period of time.
While changing beaters, the hand making the change will be entirely unavailable to play notes. So, for example, if a vibraphone were to play at the same moment that one of the hands was changing mallets, it wouldn’t be possible to play more than a two voice chord. Whenever writing mallet changes, imagine yourself doing the change in real time, and see how frantic it seems. Beater changes should, when possible, be relaxed and comfortable, with a low probability of knocking everything over in the process.
This is really a matter of opinion (isn’t everything?) but I strongly prefer to see a simple note in text rather than graphics like these:
(I added the text for clarification; the symbols are usually just added on their own)
For reasons that I can’t really explain, these images just do not compute in my head. Also, they don’t provide the (sometimes necessary) clarity of which hand should be picking up the new beater. I also can’t stand seeing instrument changes in this way, but that’s for another post.
Holding more than one beater per hand is always going to make things more difficult. There is nothing natural about holding more than one beater in a hand, but here we are. One example of this is doing a buzz roll with sticks while holding additional beaters. Proper buzzing technique won’t be possible, so the resultant sound will more than likely have a bit of a disconnected quality as opposed to the usual continuous buzz.
Multi-voice chords on a keyboard could be altered in a few ways by this. First, voices could just be omitted, so that whatever is played sounds homogenous. Or, you could have all voices played regardless, which could result in some pretty cool timbres.
Finally, some sounds just don’t work. Not everything is meant to be bowed, brushes just don’t get sound out of some instruments (including most pitched percussion), etc. It is, of course, subjective to say something does or does not “work”, but some combinations such as the aforementioned seem to be unanimously disappointing. But, there’s no harm in trying everything!