So I got it working with Daytona++ (pinned thread in this forum).
Just set the emulators to master/slave, twin cabinet, and proper car numbers and it launched right into attract mode. Daytona++ sets it to "Free Play" and it says "Press Start to Play" - which leads me to believe that it shouldn't go straight into a game. Daytona++ also enables the panoramic attract mode on a twin cab normally only seen in 4+ linked cabinet setups. No idea why Sega disabled panoramic attract mode of all things on twin cabs.
On another copy of M2 emulator without Daytona++, I deleted all the NVRAM files, set the emulator to single mode, and it loaded into attract mode as well, but said "Insert Coin" as expected. I'll mess around with a linked copy of regular M2 emu and see if I can get attract mode working.
I haven't had a lot of time to work on my twin cabinet lately, but I did get the marquee lit up. The stock light was a weird 3ft fluorescent tube. I've honestly never seen a 3ft tube - only 2ft and 4ft. Lowes had a 3ft single bulb LED shop light that fight perfectly, so I swapped it in. I think its too bright honestly. Something else to mess around with.