How to pass id of a related entity record to edit web form step

I have a web form that has four steps. Step 1's mode is Edit and targets entity A. Step 2's mode is also Edit and targets entity B. Entity A has a lookup to entity B.

I dont have any issue in Insert mode as it creates a new record and links it.

To edit the record, how do I specify the record source to edit for step 2? The options I can see for Source Type are Result From Previous Step and Query String.

It doesn't work when I use Result From Previous Step (because previous step reference is Entity A).

It doesn't work when I use Query String (because it passes the ID of entity A record).

Is there a way I can dynamically change the id of the query string when next button is clicked(by getting the entity B id from the lookup) ?

Any suggestion for doing this?

Step 1 - Entity A - Edit (A has a lookup to B)

Step 2 - Entity B - Edit

Step 3 - Entity A - Edit

Step 4 - Entity A - Edit

Thanks,
Gopi