How can I split a shower head with this fitting?
Most hand held shower hose/sprayer units utilize 1/2" pipe thread, aka 1/2"NPT (not all, some have a proprietary thread). You should be able to rig a tee to that mounting bracket (somehow, using hose clamps, velcro, ingenuity, etc.) using 1/2" PCV fittings.

You could make it so the hose supplies the fixed tee, then use an additional hose or other fixed mount to locate two heads wherever you want.

Be advised, PVC pipe fittings use a tapered thread, which seals by taper along with thread compound and/or teflon tape. Those shower hoses and heads that would have the same thread (1/2"NPT) would utilize a non-tapered thread which seals by use of a flat rubber or vinyl washer/gasket. Does this mean that they will not work with PVC fittings? No, it means they will seem to not thread in very far perhaps, and to get a seal you made need some combination of teflon tape and gaskets. It's an open-ended system so it should not be under any real pressure anyway. Good luck.