For 6mm2 T&E, I'd most likely use separate 20mm oval conduits, a single for each of The 2 cables, but I'd guess you would require 32mm oval conduit it you truly needed to put both equally in a single size. To be a sparky I might use what I'd in https://minitrunking03345.tribunablog.com/a-review-of-oval-conduit-41744221