From 'A Commentary on Le Blanc's Defense de La Viole', by Barbara Garvery Jackson, VdGSA Journal Volume 12.

'Here he feels the fretted instruments have a definite advantage in the playing of duets - that it is impossible for two cellists to play in tune with each other but that two gambists, so long as the strings are not false, can play beautifully in tune'

'He' is Hubert Le Blanc.

But what about one of each?