That would depend on what you want to achieve. If you're aim is historical precision, you would probably stick to the tenshos for 左能 as listed in Setsumon Kaiji 說文解字. Writing 佐 as 左 was the rule of orthography when tensho was at its peak.
But if you prefer the compound form 佐 at the cost of historical precision, then you could make a stylistic adaption and combine the tenshos of 人 and 左 and consider that an approximation. It would actually become a character of reisho composition written in the tensho fashion.
So basically you're choosing from 1) historical tensho script and 2) tensho-style script.
Will these go on a seal ?