The pair were outstanding at Vitesse last term.
"Vitesse were pushing for automatic Champions League qualification; they fell a bit short in the last few weeks but still qualified for the Europa League," Holland told chelseafc.com.
"Patrick and Tomas played more or less every game in full-back positions. Tomas had the odd game at centre-back but played at right-back for most of the season. They beat Ajax, the champions, home and away, and they've done well against the other top clubs.
"Patrick has matured as a player and person; it's been the best loan period of his career to date. Tomas could ultimately become a centre-back, but he's been able to develop his football at right-back, where he has to get up and down and work the ball a bit more."