Aspects of the relationship:

Serials A and B begin as two separate publications; serial B becomes part of serial A; serial A retains the same title (if the title changes consider the relationship to be a merger rather than an absorption)

Linking entry fields:

Serial A: one field tagged 780 X5; the date of the issue of title A that was the first to include title B may be given in subfield $g

Serial B: one field tagged 785 X4; $g may be given for date of issue absorbed

See also:

14.2. Chronological relationships (fields 780/785)