There ARE router motors which turn in either direction, however, they are specifically built for use on pin routers and CNC routers - those are also the tools that LH bits are made for. As to two cutters running in opposite directions, that is also exactly how industrial parts copiers (the sort of tools which are used to produce flat chair legs, etc in the furniture industry) work - the workpiece is attached to a template which runs between two vertical spindles which are pneumatically held onto the template. Biggest problem with industrial LH motors is that the tool holders, collets, etc are only are made for RH rotation so they need to be clamped up very tightly and checked regularly.
If an 18,000 rpm motor can't be found why doesn't the OP consider the approach once used in smaller industrial overhead pin routers, a 3000 rpm (in USA 3600 rpm) induction motor with a drive pulley attached, a flat belt and a second spindle with a larger pulley on top. The circumference ratio 1:6 would give a spindle speed of 18,000 rpm (21,600rpm in the USA) which is adequate for many tasks and which is about as fast as you can go on steel bearings (24,000 rpm really needs ceramics or other technology bearings)