goodmorning,

ok... now I understand better. This is an example. I chose the space group 73 because the 78 has a mirror plane... I don't know which kind of lattice do you want to simulate ...neither if the two lattice (BeO and MgO) are commensurable ...but the idea is to build a two-layers slab, one layer of MgO and the other layer of BeO and then roll the slab
The lattice parameter is chosen randomly ...

Title
SLAB
73
3.47
4
8 0.5 0.0 0.000000
4  0.0 0.0 0.0000
12 0.5 0.0 2.200000
8  0.0 0.0 2.2000
SWCNT
10 10
TESTGEOM
END
END

the ZnO@GaN were done exactly in this way... if you roll up a two-layers slab you end with a double wall nanotube