Yes, in theory you can reach 20Gbps using the LAG but in real world that never happens because the traffic can rarely be splitted perfectly balanced. A single TCP session will only use one of the LAG interfaces at 10Gbps.
If those switches are SG300 series, you need to add vlan 2 as tagged on the trunk links. It's not enough to configure the link as trunk. Go to VLAN Management -> Port VLAN Membership and check that the interface (GExx) that connects both switches is...
You don't get 20Gbps, you get a LAG (Link Aggregation Group) of 2x10Gbps. A packet will be sent via one of those two 10Gbps links at 10Gbps speed. With a LAG you get redundancy and some sort of load balancing.
You are trying to connect a 10G SFP+ DAC into a 1G SFP slot. The CBS350-8T-E-2G has 2 units of 1G SFP slots while the other switch, CBS350-8MGP-2X that works, has 2 units of 10G SFP+ slots. Your 10G SFP+ DAC will never work with the CBS350-8T-E-2G.
