What size of comforter should I use if I have two twin size mattress sit side by side ?
In general two twin sizes mattress sit side by side will form a king size bed through it is not perfectly a king.
For example, if you have two standard twin size 38x75 inch mattress, then two mattress will form a 76x75 inch bed. The standard King size is 78x80 inch. So two-twin size mattress together will have same width but 5 inch shorter as King size mattress. This is a standard size example. Some twin size mattress has size of 39x80 inch. This will be a perfect standard king size.
The best policy is to know your bed size before purchase matching bedding set, specially the bedding set includes fitted sheet and bed skirt. The fitted sheet and bed skirt will need more strict size matching than flat sheet and other components of bedding set.