Hi,
To have both versions on the same site you must have the latest version of each installed. (5.10.12 and 6.8.3)
As for the cron job, both versions have a different cron url. You need to activate the cron in v6, so Acyba's cron server will trigger the cron url that belongs to v6 instead of the v5 one..
Please note that when you migrate to v6 you need to replace your modules like the subscription form with the v6 ones. Otherwise users keep subscribing to v5 instead of v6.