  We demonstrate a technique to lock simultaneously two laser frequencies to
each step of a two-photon transition in the presence of a magnetic field
sufficiently large to gain access to the hyperfine Paschen-Back regime. A
ladder configuration with the 5S$_{1/2}$, 5P$_{3/2}$ and 5D$_{5/2}$ terms in a
thermal vapour of $^{87}$Rb atoms is used. The two lasers remain locked for
more than 24 hours. For the sum of the laser frequencies, which represents the
stability of the two-photon lock, we measure a frequency instability of less
than the Rb D$_2$ natural linewidth of 6 MHz for nearly all measured time
scales
