  It is shown that a quasi two dimensional (layered) Heisenberg antiferromagnet
with fully frustrated interplane couplings ({\it e.g.} on a body-centered
tetragonal lattice) generically exhibits two thermal phase transitions with
lowering temperature -- an upper transition at $T_{TO}$ (``order from disorder
without order'') in which the lattice point-group symmetry is spontaneously
broken, and a lower N\'{e}el transition at $T_{N}$ at which spin-rotation
symmetry is broken. Although this is the same sequence of transitions observed
in La$_2$CuO$_4$, in the Heisenberg model (without additional lattice degrees
of freedom) $(T_{TO}-T_N) /T_N$ is much smaller than is observed. The model may
apply to the bilayer cuprate La$_2$CaCuO$_6$, in which the transitions are
nearly coincident.
