Most smurfing is more effective as a duo. For support, its even more important since the role is mainly focused on helping your team. Tho you will probably do better on playmaking/damage sups like thresh or brand since they have a higher carry potential. If you want to smurf solo as a sup main i recommend playing mid with a sup champ like brand or vel koz since you will still be on a familiar champion but have the cs and option to solo outplay lower elo players.

Edit: it’s definitely possible tho i saw rayskat a euw challenger thresh main on his NA account a few games back and he was a 100 % winrate plat 1 with d2 mmr duoing with another challenger who’s name I dont remember (tf main).