Two transaction variant do not working for a user

Hi Gurus

I have created two Z t code and made transaction variant through T Code SHD0 and assigned these two ZTcode to a user. now when a user is running the Ztcode screen variant is active on only one. when i assign second one to the user again than screen variant of first ZT code do not work.

can any one tell me why system is doing so.

Regards

Shiv