Arthur knows. Merlin knows Arthur knows. Arthur knows, Merlin knows Arthur knows. Merlin knows, Arthur knows Merlin knows Arthur knows.
Not that either of them haven't tried to discuss it...
"Merlin…" one would say.
"Yes, Sire?" The other would respond.
They would both look at each other.
Awkward moments would pass, then the blonde would throw something towards the other's head.
Both of them knew... not that they were going to talk about it anytime soon.
Even in moments, such as these.
"We are stuck in a cave, with no way out," the royal would observe. The other would commend him on his recently improved IQ, dodging a random object thrown at his head.
Then after, the finer dressed one had pointed towards the collapsed entrance, he'd signal the other, before turning away.
As if, by chance, a large flash would fill the cave; the entrance would miraculously be cleared.
They would both look at each other in moments like these. One would smile. The other would grin, then the former would roll his eyes.
Merlin knows, he knows.
Arthur knows, he knows he knows.
... and as long as neither, had the desire to injure each other's pride by talking about it, they could both, continue to safeguard this knowledge.
"I know, Merlin." Arthur finally said, one day.
"I know, Arthur," replied Merlin.