I'm running an XT targeting existing customers, but the remaining audience that isn't captured (who i'm assuming are new customers) I want to fall into a 50/50 split A/B test.
Is this possible? I have an XT activity and A/B activity running separately trying to achieve this, but the A/B test just isn't firing.
This is possible. Though you many need to define a "counter audience" to get it working properly. For example, this setup should work--
You can set up and AB and XT as well on same page.
You just need to make sure that your XT doesn't impact result of your AB test.
For eg. If you have changed CTA color in AB test and you have changed CTA text in XT test, you won't be able to determine whether that XT was reason for increase/decrease in conversion or the AB.
So i would recommend you to use specific set of Audience for your AB test and exclude those audience from XT.
I hope this will help to resolve ur query.