What unlocks the new fertility potion scene for Hretha and Infrith?

[MB_The_First]

Title. Couldn't find the option on either of their menus. Do you need to have had a child with both of them beforehand?

 

[Tarnakus]

 (infrith.totalKids() > 0 || hretha.totalKids() > 0)
&& !hretha.isPregnant() && !infrith.isPregnant()
&& pc.hasRealCock() && !flags.HRETHA_TONIC_EVENT

You need to have at least one child with one of them, neither of them should be currently pregnant, you need to have real cock and you didn't saw the scene yet.

If you saw it you can repeat it from Hretha sex menu granted again neither is pregnant.

For pregnancy check:
Hretha even before she tells you about it: in both Fuck Her and Get Ridden - she implies she is already pregnant - in later there is Lumia bit in first the "sire of my children" bit I think - parsers in the scenes are a little inconsistent.
For Infrith Quickie scene is block before she tells you.