One handed swords used with both hands
The Codex Manesse, Manesse Codex, or Große Heidelberger Liederhandschrift (Heidelberg, Universitätsbibliothek, Cpg 848) is a Liederhandschrift (book of songs/poetry), the single most comprehensive source of Middle High German Minnesang poetry, written and illustrated between c. 1304 when the main part was completed, and c. 1340 with the addenda.
Having brought down a giant by cutting his lower legs, a knight slays him with a two-handed sword blow. Marginal illustration from Add MS 24686, 17r, dated 1284 – 1316. The fully mail-clad knight wears a gilt great helm and has his shield slung on his side, suspended from a guige strap. Thus he can strike the death blow with two hands which is shown frequently in single combat of the period.
Miniature from Jungfrauenspiegel (Virgin's Mirror) of about 1140 (illustration c 1200), Germany, shows two-handed sword use and grappling, shields on the back. It is an allegory, so we have to be cautious regarding the reality of combat. Particularly the caved in helmets have been subject to debate. Early and high medieval helmets and mail are made from iron, not steel. They were softer than in the late Middle Ages. It might not have been impossible to penetrate high medieval armour.
Some more examples of two handed sword use. Berlin, MS. Germ. 2°282, Eneit by Heinrich von Veldeke, fol. 53r (Thuringia, 1210-1220 ) Heidelberg, Cod. Pal. germ. 848 Große Heidelberger Liederhandschrift aka Codex Manesse, fol. 321v (Zürich, 1300 – 1340) St. Gallen, Kantonsbibliothek, World Chronicle of Rudolf von Ems, VadSlg Ms. 302, fol. 109r (Upper Rhine area, 1310) Swiss manuscript from about 1340
A remarkable miniature dating from the first half of the 13th century, showing an armoured horseman with his shield slung on his back, wielding his sword with both hands. This is the earliest representation of two-handed sword use that I am aware of, that is not merely a death-blow but an actual fighting technique. (Gottfried von Straßburg’s ”Tristan & Isolde“, BSB Cgm 51, f. 86v)