Unfortunately, switch can't be used to connect multiple EtherCAT chanins to a single computer, but there are other options to do so.
1. Use computer which has more then one Ethernet port:
Every chain will work as individual which allows you to use full 100Mbps on each.
In this configuration, the EtherCAT chains needs to be synchronized to each other, so every chain needs also a Sync junction.
2. Use the Beckhof EtherCAT junction CU1128:
In this case, it is possible to use multiple chains, but the speed will be limited to the speed of single chain. The chains will behave as one, so no aditional synchronization is needed.