Two primary keys to one foreign key
I would suggest that you either want a single "couple" table instead of two, if the properties of each coupling are sufficiently similar, or a couple table that the two existing types of couple "inherit" from, like so:

RC                  Couple                                Egg
----------          ---------------------------           ----------------
C ID (FK)   -----&gt;  C ID (PK)                    &lt;-----   C ID (FK)
(rc props)    |     (generic couple properties)           (egg properties)
              |
BC            |
----------    |
C ID (FK)   --'
(BC props)


You could still have breeding and racing details for any could in this arrangement (which might actually be fine, of course) but each egg can only be associated with one couple. You won't have problems with overlapping ID ranges as there is only one range of IDs for couples when modeled this way, and all other tables with data relating to couples only have the one FK linking to the could record.