I only use 1 gmail account, so I can't verify this. It's actually from the Evo forum.
Basically you'll need to run the phone's setup (all applications -> Setup) again and enter the desired gmail account when it asks you to sign in. That will then be your primary account and be used for syncing.
Also, there might be another option. I just came up with this myself
press the "menu" button
Press the "menu" button again
select "Account settings"
in the manage accounts section, select your primary account
de-select the "sync calendar"
then go back, select the other account and select "Sync calendar".
EDIT : yes, I know. I like to make things complicated