You CAN have both be able to have ACCESS to the drive. However, it is NOT true that one can be read+write and the other read only. The only way this is safe is if one system is NOT using (and if possible, seeing) the drive at the same time. Why? Because manipulations to the drive can happen from the system that has read+write including modifications to meta data areas... and if that information is not communicated somehow to the other host, then even if it's supposedly "read only", you'll be messing up the internal representation on that host and possible cause issues, perhaps major issues.