You can appear for any module at any time after completion of 9 months from the date of registration (Without fine, under normal circumstances).
It's completely your own decision whether you appear for both modules at a time or any one module. However, fees would differ.
One module exams fees would costs you Rs. 1,200/- and both module would costs you Rs. 2,400/-.
I would suggest you to prepare and give both the modules at a time.