Q:
Ascended ring question about "uniqueness"
A:
Yes you can use an unupgraded ring and an upgraded ring at the same time of the same type, though its not exactly easy to upgrade them.
Can’t I just use the infusions from the laurel vendor?
nope by infused they mean the upgraded ring variant that comes with 5 AR on it before you slot the extra infusion.
You have two types of infusion, you have the infusion your thinking of which is basically the same as slotting gems into exotic or lower trinkets, then you have the upgrading infuse that changes the item into a “<ringname> (Infused)” variant that has 5AR before being slotted.
To upgrade a ring to the “infused” variant (yes I know its confusing, blame Anet) you need to put the ring in the mystic forge with 1 Shard of Crystallized Mists Essence (from level 20+ fractals), 3 Glob of Coagulated Mists Essence (from 10-19 fractals) and 5 Vial of Condensed Mists Essence (from 1-9 fractals)