Zanac X Zanac is a combination of two iterations of Zanac, the original Zanac with three separate versions, and Zanac Neo which is the sequel to Zanac. It was released for the PS1 in 2001 by Compile to celebrate Zanac's 15th anniversary.
Zanac X Zanac presents three separate versions of Zanac, each one slightly different. There is the original NES version, the Famicom Disk version, and a special version exclusive to Zanac X Zanac. There is also a Score Trial Mode, that tests the player's skills on certain scenarios.
This one is an exact port of the original NES version. Nothing from that one was changed, but there is a setting called ROM Plus which increases the enemy bullet limit.
This is the Famicom Disk System port and has several differences compared to the NES version.
This is a unique iteration of Zanac that is exclusive to Zanac X Zanac and is different compared to the ROM and Disk versions.
This would be the sequel to Zanac which takes place some time after System 1 is destroyed. It keeps with the main style of Zanac but also adds some other things, in addition to graphics.
There is also a feature that keeps record of certain feats that are done while playing the games, such as "No Death" or "Get all Eye-Eyes". This is similar to R-Type Delta which had a similar system.
Zanac X Zanac FAQ - A through FAQ on Zanac X Zanac.