You can achieve this using ‘Merge Datatable’ activity. Read data from both the input files, pass on the Datatables to this acitivity to merge into one Datatable and then you can write the output Datatable in the output file.
If your output file is your only “constant” , i would do something like this
step 1 - read the output file template / create datatable w/ all the needed columns
step 2 - read file 1 to datatable1
step3 - for each row of datatable1 use activity assign (or multiple assign) to assign the columns to variables w/ column name
step4 - add dataRow with the array created by all the variables you need for your output_datatable
You could also skip the variables part in step3 and assign directly to the output datatable , but for starters try it like this.