First of all, it shouldn't be a problem setting them both up as CS.
Second, as others have said, you'll have performance issues when trying to access your DVD burner and the ZIP drive at the same time.
Third, the performance is worse when transferring data between the DVD and the ZIP drive. The data need to go to the IDE controller first, then it goes to the destination. In the meantime, the source device is just sitting there until the data is written on the other device and the bus is clear. This is why SCSI was awesome back in the day. In a SCSI bus, data is transferred directly from device to device on the same cable. Your system remains responsive, and there's no waiting for the bus to clear.
As bad as that sounds, it's what we had to deal with back in the day.
I remember having to put each HDD on it's own channel if I wanted the best speeds. The optical drive gets put on the 2nd HDD's channel to not slow me down if I install software froM CD to C:. I also have to remember to master my CDs on the C: drive before burning (otherwise potential coasters). Having to keep the devices and what channel they are on in mind when burning CDs was just a part of computing life in those days - at least for me. 😀