  We report on the magnetic merging of ultracold atomic gases of $^{85}$Rb and
$^{87}$Rb by the controlled overlap of two initially spatially-separated
magnetic traps. We present a detailed analysis of the combined magnetic field
potential as the two traps are brought together which predicts a clear optimum
trajectory for the merging. We verify this prediction experimentally using
$^{85}$Rb and find that the final atom number in the merged trap is maximised
with minimal heating by following the predicted optimum trajectory. Using the
magnetic merging approach allows us to create variable ratio isotopic Rb
mixtures with a single laser cooling setup by simply storing one isotope in a
magnetic trap before jumping the laser frequencies to the transitions necessary
to laser cool the second isotope. This offers a simple and cost effective way
to achieve suitable starting conditions for sympathetic cooling of $^{85}$Rb by
$^{87}$Rb towards quantum degeneracy.
