Hey giobox nice work there. I guess all that digging noise I've been hearing in my Porsche CAN rabbit hole is you nearby in the Miata hole.
A word of caution on the splitter. This can work so long as both devices are using separate wire pairs on the connector (for example CAN +/- pins 6/14 and ISO +/- pins 7/15. But when I attempted the dual-TX on my Porsche Cayman which does OBD over CAN so only supports pins 6/14 I saw very dodgy and unreliable behavior. I bought several different splitters and Y cables before really digging into the CAN bus electrical specs. You may be aware of all this, but it's super easy to blow up the impedance on a CAN line.
The other way is have one device in pure RX promiscuous mode (great for CAN) and use the other for TX. I'm doing this right now doing frame capture on both sides of the OBD-to-CAN gateway to study injection of OBD signals directly onto CAN.
Regarding built in tire pressures. I figured these out on Porsche Cayman and then spent time doing detailed analysis of how inaccurate they are. See this post in an active thread we have on CAN hacking over on 718Forum -
[https://www.718forum.com/threads/successfully-hacked-my-718-gts-can-bus-for-racechrono-data.31918/page-4?post_id=458942#post-458942]
Bottom line is these OE sensors are poor in both temporal accuracy (X axis) due to need to preserve battery life of the stems, and crappy precision (Y axis) due to need to buy the cheapest possible sensor. Suspect my results apply to most if not all other OE TPMS stems - I found the minimum update rate be 15 seconds, and minimum pressure accuracy to be 1.5psi steps. This is fine for daily driving but useless for track applications. RaceChrono supports BT track-grade tire pressure systems with realtime updates - I went with these and have been very happy [https://team-plus.com/product/tyre-pressure-monitors-tpms-track/?v=0b3b97fa6688]
