The OP wants to crimp two conductors into one terminal, such that the total conductor area of the two is within the range suitable for the terminal.
If the ring terminals are of good quality, correctly crimped and securely screwed to the plate, the difference in reliability between two stacked rings and one ring with two cables should be trivial. But there are certainly situations (for me, not so much in installation work but within equipment) where it is helpful to crimp two into one and I sometimes do this in situations where a slight reduction in mechanical retention of the cable insulation (not conductor) would not be an issue.
Care is needed to ensure all strands of both cables go right to the end of the crimp tunnel; I like to strip them, cable tie them together with the ends of the insulation aligned so that they behave as one, and then trim the conductors to length. Depending on the type of terminal and insulation thickness, the entry funnel sometimes does not have room for the two cables side-by-side due to the extra layers of insulation. Obviously to make a satisfactory job the insulation must enter as far as possible, so it is sometimes necessary to form the entry into an oval first.
Actually last night (or this morning, I didn't finish Sunday's work until 2am Monday) I used a 25mm² copper tube butt crimp to connect six 4mm² stranded conductors to one 16mm² for some DC distribution in some mobile plant. Same deal, strap all six into a bundle with the insulation exactly level and the conductors over-length, gently form up the conductors together with a very slight twist, then trim to length and crimp. The 16mm² I stripped double-length, cut 10mm² of the strands to length and folded the rest back, to fill the terminal. Doubling a complete conductor back so that it loops through unbroken is nice in theory but difficult in practice, because the fold point bulges out at the sides and is larger than the sum of the two conductors.
In general, I would say care and skill are needed to ensure any non-standard deployment of crimps is sound. It is probably forbidden by the NEC (US wiring regs) where there is a specific requirement that only one conductor is placed in each terminal unless specifically designed for two.