It is very simple, just select the field to which you want to show two decimal places. Right click on the field go to format field-->go to number tab -->click on customize-->then in Decimal select "1.0".
http://www.crystalreportsbook.com/Forum/forum_posts.asp?TID=9338
Second option in formula , you can give format, so it will take 1 decimal.(check in above link)