Rear 2 pc rotor are available.
Unlike rear rotors typically have a drum e-brake, since mid 2010 most of the parking brakes are now integrated to ePB (w/single piston floating caliper) so the upgrade/oversize are more restricted as you know unless the parking brake is deleted which is not recommended.
So far we only offered rear BBK (retaining OE calipers) for certain popular motorsports cars such as: Tesla M3 & S, Supra 2020+ MK5, Corvette C8 (with a 3rd parking brake) where there is a demand and space (min. 50mm in rotor diameter oversize) is allowed.
Since ST Perf. rear rotor is a 355mm so I think it's a decent size w/o requiring over-sizing, as we have seen OE fronts are 400mm+ while the rears are still kept at 355/360mm (eg. Trackhawk 400/350, Mercedes GT R CCM 420/360)