Operating systems have to have their own drives or partitions, so, unless you are prepared to re-partition the drive into at least two partitions, it is not possible. Also, I believe I am right in saying, '95 needs FAT16, I'm not sure if it ever supported FAT32.
If you don't have something like Partition Magic (PM), which will do the job for you without loosing data, you will loose all the data on your drive. Even with PM, you would be well advised to back up any important data first.
Having said that, I have never used this combination, and am not sure whether '98 will let you keep '95 in tact. I have a feeling it wont! The way to overcome this, is to download one of the many free boot managers available, or if you have PM, use it's boot manager. This is a small piece of software, that allows you the option to boot into either system.
The golden rule is, you must always load the oldest operating system first, so '95 first and then '98.