As musicians there's always something that we can practice, but as percussionists the task of practicing can seem overwhelming at times. I often thought, "How am I going to practice all of these in time for (blank)?" This question arose when I started watching the television show Avatar the Last Airbender. The premise of the show is that a child, Aang, has to learn and master four different fighting styles, in only one year, in order to save the world from an evil conqueror (very very good show). Obviously, we don't have to save the world after we practice (at least not in the same sense), but sometimes it can certainly feel that way. So, I watched this show seeing myself in the character, and I was genuinely interested in how he would accomplish this daunting task.
The key for Aang's "multitasking" was to find as many common threads between styles as possible in order to make learning, mastering, and switching between them easy and automatic; in some cases he had to find opposites as well in order to understand certain styles better. For us, our common thread is the snare drum and by incorporating the most basic elements of stick control in everything we do, it will make practicing multiple instruments a lot easier. In my case, I practiced stick control obsessively because I believed in that principle whole heartedly, and even as I practiced other instruments, I tried to make it feel as if I was just playing snare drum. I'll describe this thought process starting from the "closest" techniques to snare, to the farthest.
Mallets were where I really started to use this idea. Its hand to hand motion directly stems from stick control, and the main difference between the instruments is the rebound and horizontal motions. Here the fundamental idea that makes mallets easier is the full and up strokes, getting off the bar equally or faster than you arrived to it. So, I would practice my up strokes, first thinking about the snare drum, and then thinking about the mallets. The more comfortable you become getting to and from the bar, the easier it will be to move around the instrument, essentially by not taking too much time in any given spot or position. From here, you now have stick control, a different feel of the rebound, and horizontal movement at your disposal to use.
You can now apply both of these to timpani. Again, stick control can be a starting point, but the rebound is......weird. It's not as "dead" as a xylophone, but not as bouncy as the snare drum; it's often in between the two. Also, the horizontal movement can be applied, though this it's on a much wider scale, and includes the rotation of the torso, not just the arms. Hopefully you can start to make more connections like this across all the instruments, but the starting point can always be stick control. As you warm up, use this time to go through the "different" sides and feels of the instruments, so when you do get to them you won't feel like they are a completely new or different task. Yes, they all have their individual challenges and traits, but they all are based around the same principles: approach to the instrument (character), preparation of the stroke (to get ready to play that character), the stroke itself, contact to the instrument (length of time the object remains on the instrument), follow through (the body's response to the impact), rebound (the object's and instrument's response to the impact), and the reload (for the next prep).
For me, I spent a long time figuring out variations of these principles on snare drum and put together combinations that would work out for other instruments. The better I got with, the more I would do variations based on any instrument and transfer that information to others, and the cycle would go on. Before long, every instrument influences the next in some way, and a place to try this is snare drum and timpani or xylophone. I found that playing snare drum as if it were a timpani helped free up my playing and my variety of sounds, which almost automatically helped all the others. So, I didn't have to spend an hour working on a tambourine stroke, but maybe I spent 10 minutes translating the information into "tambourine." Also, since timpani forces you to use arm to get around, cymbals can benefit from this as well. Granted, the mallets are not 15lbs+, but the engagement of the arm is key for cymbals to work well. Paying attention to that and the strokes that produce good sounds on timpani, generally full strokes, up strokes or extremely bounced down strokes (not a dead, harsh throw), gives you a sense of what will work for cymbals.
In a way, you are multitasking without jeopardizing any individual instrument's growth, and by doing this each one can develop more equally. It often feels like we put so much stock in snare drum or mallets and when we go to our technique bank, tambourine and bass drum are pretty much empty. I'm definitely suggesting to not think of each instrument as a separate subject, even though on some level they are. Four mallet technique can influence your tambourine roll, your snare drum roll can influence your cymbal crash, and your timpani playing can influence your xylophone. Definitely set aside time to literally practice each instrument (maybe doing the strongest and weakest instruments back to back), but keep in mind as you do this that, within one of them, all the others reside. Happy practicing.