Forms and embedded reports

Perhaps instead of having 2 forms in the child-table; you have 1 form and use Form Rules to control which data-set's are displayed based on which relationship (parent) is populated; this way you do not have to control every report to show one form v. another form in the same table because of the 2 different parents. 

This is a very common function of a child-table to have more than 1 parent; but only data from 1 parent be visible at a time depending on the relationship.

Otherwise, you will run into issues with every report; and users who build their own reports may not be able to see the data they need because they do not have the ability to view the correct form you have designed from their report.

Typically, you will put parent 1 lookup fields in 1 section heading group; and then put the 2nd parent  lookup fields in a second section heading group.

Then use 2 form rules - when [Related Reference Field 1] is not empty, show Section 1.  When [Related Reference Field 2] is not empty, show Section 2.  If users are adding to this table from the table (and not from either parent); then you will need to show the 2 related reference fields together on the form for the user to select one or other parent first.