Interlining baggage on split-class itineraries

[cinereus]

Let’s say I have a booking that goes MEX-MAD-LGW on IB.

MEX-MAD is in business and MAD-LGW is in Blue.

1. Can I interline hold baggage from MEX to LGW?
2. How does this work for allowances if I have 2 hold bags for the first leg and only 1 for the second leg?

[SamG]

If it’s the same booking then you’d usually get the allowance of the longest leg