No, it is not required to perform sector erase if you are programming page 1 first and page 2 after some time. Sector erase is required only when you re-program an already programmed page.
In flash memory devices, program operation can change a bit from 1 to 0. It cannot change a bit from 0 to 1. Only an erase operation can change the bits from 0 to 1. So, as long as you are programming erased memory locations inside a sector, it is not required to perform sector erase. If you want to change data already programmed into the flash, then you have to perform sector erase. Please note, a sector erase operation will erase all pages inside a particular sector. If you need to change data only in one page, you have to back up data from other pages before performing sector erase operation. After sector erase operation is finished, program data back to flash.
I hope the above information answers your query. Please feel free to ask if you need any clarifications.
Thanks and Regards,