In Sonic 2 there is a blue bar separating the screen of each player.
From what i read in the past (but i don't find the documentation anymore) this blue bar is generated with a long write access to vdp.
The first word access disable display and start dma. Because display is disabled dma transfert rate is maximum.
Because dma start the 68k give the bus to vdp and have to wait vdp release the bus to be able to write the second word (So only after dma ended).
The second word active display.

Correct rendering is :