Double window roller

This looks like a Coil Balance system, where spring mechanisms within that housing regulate the opening and closing of the hung window. Specifically, it seems you have our 15-102 Double Carrier. See the accessories I've listed on this page. This system uses Series 145 Coil Balances, sold separately. If your coils have snapped you'll need to replace them. Coil balances come in different spring weights, which indicate the weight each individual spring can support. Look for stamps on the springs, usually near the tongue, to determine what spring weights you need. If there are none or you are unsure, weigh your window sash. Give me that exact weight and I can work out what you need.