Passive xover for a sub. How to?

Quick question. Hopefully this is posted in the correct place.

Is it safe for the stereo amplifier, to run an 'off the shelf' type pair of 80-100Hz passive crossovers, run the HPF outputs to the L &R speakers and then sum (join) the 80-100Hz LPF outputs to a single passive subwoofer?

Was thinking of doing this for a cheap garden system setup, just for background low listening levels.

Is it safe? Good/Bad practice?

You can't do it and expect the power amplifiers to survive. Only under special design conditions of the amplifiers, or large coupling cap isolation, can you even do it after guaranteeing that the channel to channel bass signal and amplifier gain is identical. DC coupling of the usual consumer electronics stereo amplifiers together even with no signal can drive large currents between them and destroy the output stage. You may possibly get away with the general idea by using a dual coil woofer.