Is there a way to facilitate this with lightning so I avoid becoming an intermediary proxying funds and instead a single invoice translates to 2 (or more) peer-to-peer payments?
yes and no, yes with bolt12, no without but you can use the keysend to send a payment
This is how I am implementing tipping a plugin developer in core lightning (See)
The basic idea is that you get the payment of X and then in your lightning node you intercept this event and pay the node that you want to may with a fraction of this payment with keysend.
With keysend you can pay a node with a public key and not with an invoice, ofcs if all the node implements bolt12 you can use a bolt12 invoice with any amount instead of a keysend
Do this in core lightning is trivial I do not know with others implementations