if {[catch {package require topotools 1.1} ver]} {
   vmdcon -error "$ver. This script requires at least TopoTools v1.1. Exiting..."
   quit
}
if {[catch {package require pbctools 2.3} ver]} {
   vmdcon -error "$ver. This script requires at least pbctools v2.3. Exiting..."
   quit
}

set solute [mol new leftside.pdb]
set solvent [mol new rightside.pdb]
set combined [::TopoTools::mergemols [list $solute $solvent]]
set all [atomselect top "all"]
$all writepdb combined.pdb
topo writelammpsdata data.composite atomic