plotting model from two compute options fails

[kecnry]

script to reproduce:

import phoebe
b = phoebe.default_binary()
b.add_dataset('lc', compute_phases=phoebe.linspace(0,1,101), dataset='lc_primary_only')
b.add_dataset('lc', compute_phases=phoebe.linspace(0,1,101), dataset='lc_secondary_only')
b.add_compute('phoebe', 
              distortion_method={'primary': 'roche', 'secondary': 'none'},
              enabled={'lc_primary_only': True, 'lc_secondary_only': False},
              compute='comp_primary_only')
b.add_compute('phoebe',
              distortion_method={'primary': 'none', 'secondary': 'roche'},
              enabled={'lc_primary_only': False, 'lc_secondary_only': True},
              compute='comp_secondary_only')
b.run_compute(compute=['comp_primary_only', 'comp_secondary_only'])
print(b.filter(context='model')  # does show the arrays are populated correctly 
b.plot(legend=True, show=True)  # raises an error currently

(see #839 (comment))