This Cisco module QSFP-40G-SR-BD is Bi-Directional (BiDi) 40Gbps QSFP, which means a single link used for RX & TX, each link 20Gbps, two LC links in total gives 40Gbps connection. If we will use only a single link instead of two, will that QSFP work on 10/20Gbps speed on a single link or it will not work at all?
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Probably, you won't be able to specify nor auto-detect "speed 20000" on the QSFP+ switchport, and as long as not all 4 lanes of the PHY return an "up" signal back into the port, line protocol for a 40G port will stay down.
The transceiver might support 4x10G breakout mode - in that case, you might be able to define a 2x 10G Etherchannel across one strand of fibre. However, coming to think of it, support for 4x10G on this transceiver variant is unlikely: After all, how would such a 4x10G breakout be implemented physically? One would need an optical demultiplexer somewhere in the fibre path...