How do you summon a zombie on a zombie horse?

There is no single command to accomplish this task in Bedrock Edition ₁. However, with that in mind, it can be done with multiple commands.

Note: This answer assumes you're aware of how the summon and ride commands work.

Summoning Jockeys

The summon command can only be used to summon specific jockeys ₁ which does not include a zombie jockey:

Only some jockeys can be summoned via commands: Skeleton Trap and a Ravager being rode by an Evoker, Pillager or a Vindicator.

_{Note: This quote has been paraphrased for the sake of brevity. For more complete information, please check the wiki article on summoning jockeys.}

Break it Down

To accomplish your desired result, you'll need to summon a zombie horse, followed by a zombie, then finally execute a ride command targeted at the two:

summon zombie_horse
summon zombie
ride @e[type=zombie,r=5] start_riding @e[type=zombie_horse,r=5] teleport_rider

_{1: As of 20 SEP 2021.}