Sharing a Ground Conductor Between Multiple Circuits
So you're paralleling two separate runs - a 12/2/2 and a 14/2 - from the panel, to a J-box. Beyond that they run as THHN wires in conduit.
You're concerned because you've been well-trained that cables and conduits should be tree topology not loops or bubbles - and there's a bubble between the 12/2/2 and the 14/2.   Good sense.
And that sense is particularly astute when it comes to neutrals.  You must be very careful to keep neutrals identified and segregated so each neutral is identified with its partner hot.
Remember that in individual wires (e.g. in conduit), no remarking is allowed for function: you can't re-mark white wires to be hot.  Which means... blue tape on a white wire does NOT make it hot; it only makes it &quot;the neutral for the blue circuit&quot;.
However, with grounds, the more the merrier.  Keep in mind ordinary service current travels on neutral.  Safety ground is for nothing but ground fault current, which is expected to be abnormal and transient.
&quot;Ground loop&quot; is a term-of-art relating to audio engineering, and the impacts of having audio equipment powered by an AC source in the audible range.   It's also used by airplane pilots to describe the airplane doing an accidental &quot;donut&quot; because they forgot crosswinds can affect the airplane even when rolling on the ground.