Assuming a professional performer with adequate equipment, can a single percussionist play a single stroke on both a timpani and a set of tubular bells at the same time (one with each hand)? The passage is at a moderate tempo so there would be sufficient time to "aim" the bell stroke to strike the correct note. I would like both instruments to ring indefinitely after, so there is no need to damp.
It depends a lot on the setup. What else is the percussionist supposed to play? How much space is there at the performance? How many Timpani do you have?
Keep in mind that a set of timpani takes a lot of space, and so does a set of tubular bells, and many other percussive instruments. It might be a challenge to place all percussive instruments in a way such that the percussionist can quickly change between instruments and play timpani and tubular bells at the same time.
It may be viable to hang a single tubular bell over the timpani set, but of course this would have less resonance!
But instead it might be better to simply add another player, even if it is just for this single note. Percussionists would often get one of their students to do such parts, it is not exactly uncommon.
Also if you intend to add more percussion: Due to the prominence of timpani in orchestral music you’d often see two percussionists, one doing timpani and one (or more) doing the rest (bass drum, tenor drum, snare, cymbals, kit, triangle, xylo/vibra/glockenspiel, triangle, woodblocks, ...). So adding a second percussionist would give you a lot of additional freedom.
Yes. I see no reason why they couldn't hit 2 things at once, one with each hand. The tempo of the piece doesn't matter; what matters is the length of time between notes. As a practical matter, they would probably have to set up with tubular bells behind the timpani, so they could play a high timpani + bells, or low timpani + bells, but doing both in the same passage might be awkward.
Also, generally the default position for tubular bells is dampened, so letting it ring would require them to keep a foot on the pedal. This would be awkward if they needed to resume playing timpani as normal while the bells are ringing. If the player is free to be tied up by holding the pedal, this is not an issue. Of course, there is always a high-tech workaround involving a dumbbell, but you should prefer an arrangement that doesn't require this if possible.