BroadCom 5720 Multi-Switch Connection failing to achieve full bandwidth

We have following Setup:

    2 redundant Dell 2848 Switches, identically configured and connected via 8-Port LAG on Ports 41-48.

    2 identically setup Dell R730 with BroadCom 5720. Operating System is Microsoft Server 2012-R2.

We wanted to make a fully redundant setup with maximum bandwidth. server S1&S2 port 1&2 are plugged into switch A and server S1&S2 port 3&4 are plugged into switch B.

On the server side, the 4 NIC are teamed together, on the switches the 2 ports each server is plugged in are put into LAG. Since the switches are only PowerConnect 2848 the LAGs are setup static. We only manage to achieve 2 GB/s.

SwitchA:

    LAG A1(Server1 p1 Server1 p2)
    LAG A2(Server2 p1 Server2 p2)
    LAG A3(Switch2 p41..p48)

SwitchB:

    LAG B1(Server1 p3 Server1 p4)
    LAG B2(Server2 p3 Server3 p4)

    LAG B3(Switch1 p41..p48)

    If we unplug LAG A1, we have a short drop and go back to 2 GB/s
    If we unplug LAG A2, we have a short drop and go back to 2 GB/s

All LAG plugged in again, next test:

    We unplug Server1 p1, we have a short drop and go back to 2 GB/s
    We unplug Server1 p3, we have a short drop and go back to 2 GB/s

This is where my confusion begins. Up until now i thought the multi-switch connection simply wasn't capable of bandwidth adding, but obviously with only one port plugged into each switch i once again have 2 GB/s. Yet i never had the hoped for 4 GB/s at the beginning.

When trying to find the source of the problem, more confusion arose:

    4 NIC teaming on each server, no LAG on switch, all ports plugged into one switch: 1 GB/s

    4 NIC teaming on each server, 4 port LAG on switch, all ports plugged into one switch: 4 GB/s

    one cable unplugged: drop to 2 GB/s.

I fail to understand this behaviour, maybe i am missing something simple or maybe i am simply dumb, but if you can, please explain it to me or make suggestions how to achieve maximum bandwidth with maximum redundancy.