From the cocotb github discussion page, the recommended answer is to use a simple module that wires together the DUTs you want to test.

https://github.com/cocotb/cocotb/issues/385

I recommend you to use a "harness" Verilog module. In this module you can instantiate DUTs in various configuration. From the Cocotb perspective your simulated top level will be the harness, instead of a standalone DUT.