If you summon an Ender Dragon riding another Ender Dragon they will both fly south backwards extremely quickly and do not seem to stop at any point.
Can be reproduced using
/summon ender_dragon ~ ~ ~ {Passengers:[{id:ender_dragon}]}
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Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-69881, which is a report that has not been resolved yet.
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