Welcome to the community @ssanct 

There are a few ways to achieve this.

Two lists. Create a form split test. The A variant submits to your A list and the B variant submits to your B list. You then trigger a  seperate welcome flow when a contact submits to either list.

Single list. Add a custom property to your form, let’s call that form_variant. Add a submit on both A and B form variants to this profile property with your the appropriate form_variant value e.g A_test or B_test. Both form variants will submit to the same list and therefore trigger the same welcome flow. In your welcome flow, add an intial conditional split on the form_variant: ‘properties about someone’ with a dimension of ‘form_variant equals A_test ‘. Then form A variant submissions will go down the YES branch and the B variants down the NO branch. If it’s possible that some contacts may enter the welcome flow with form_variant not being set, you should include an OR statement to send them down the correct branch of your flow. Note that custom properties will only appear in your flow settings options when at least one contact has that custom property set; so create it on your test profile first.

My preference is always to use a single list of subscribers unless there’s a compelling reason to split them across lists.

Hope that helps

Andy 

Thank you for that, I ended up going with the conditional split and it works well!