I think you 'd have to take uke's hand (the one that's holding yours) with your free hand and do the 'earth' part, then with your now-free other hand, do the 'heaven' part.
Here it is from gyaku hanmi the way we do it.
To do it from ai hanmi, you have to take ukes grabbing hand, free your own and continue just like in gyaku hanmi.
Only difference: You grab uke and lead him by this instead of being grabbed.
If I am picturing what you're saying right, then I've seen such a technique many times, but I don't recall what it's called. It wouldn't have occurred to me to consider it tenchinage without using both uke's arms, but I can see your reasoning.
Doing tenchi nage from gyuaku hanmi is quite "normal" to us. (aikikai in Germany, following Endo Seishiro/Christian Tissier) I think it's tested for 4th kyu.
We also do it from yokomen uchi nearly similar like shown here
Or from kata dori and kata dori men uchi.
So doing it from ryote dori is just one out of many possibilities.
At least in our aikido.