I have made some quick isolation calculations (dB) for the scenario of two rigs with 100W output power collocated using two dipoles mounted some meters apart on the same support but with separate feedlines.
A Preliminary conclusion is that a BPF + notch is likely necessary for interference free operation. With good filters and notch, the second harmonic will be at a 7uV level and the hash will be at a S1 level on the antenna connector of RIGB which is equivalent to a moderately strong signal on 20m with low noise. Without filters, the second harmonic will be S9+50dB. The phase noise "hash" will likely be at S3. This will likely be a significant problem.
Download the calculator here: isolation_calculator_nine_islands. The calculator is posted under a Free Beer license. You owe me a beer if you use it! Please give feedback and peer review the calculations.