I'm still not sold on the 'mingling' or 'mixing' of Sparks - I'm certain they'd remain separate at all times.

'All is as it should be' scene from Nemesis Part 2

G1 Megs' Spark is being held (perhaps controlled), but clearly separate from BW Megatron's.

I say 'perhaps controlled' because G1 Megs seems to exhibit no influence on BW Megs, unlike Starscream who took over Waspinator head to toe.

Although, you could speculate that given the bodies of G1 Prime and Megs were in stasis, and their Lasercores perhaps offline, they couldn't influence their host's bodies.

So much speculation!

It's all about coming up with a series of rules that answer the unanswered questions for yourself I guess.