The other ROM positions (U182, U183) are for ICs that have a different pinout such as the 27C220 and 27C2048.
The standard positions U180, U181 are for pinouts to suit 27C100, 27C200, etc.
You can't have two types of ROMs fitted at once, although if you were to disconnect _ROMEN to each ROM pair and fit a toggle switch so that only one pair can be active at once, then it would be possible to switch between the two. You'd need to add pullup resistors on the ROM _OE lines for when nothing is connected.
But it's a lot nicer use to use a ROM switcher module or BlizKick instead of chopping the hardware around.