you need 2 hand spear to use Lancer skills, end of story. In theory you could keep that shield in a slot and let auto weapon swap do the work for you to use Lancer skills, but there is no point investing into a one hand spear.
Also, don’t try auto weapon swap if you have a one hand spear equipped, because all of your Hoplite and Cataphract skills will be executed with the one hand spear instead of the two hand one then.
About the shield in second slot so you can auto swap for your skills, I’m not the right person to answer this, because I don’t know if empty main hand + shield will always switch to two hand spear for all of your skills (as Cataphract seems to be compatible with ± every weapon type aside of rapiers and 2 hand swords); better to have someone else answer this specific issue.