[quote=@Bugman]
Before I express interest, I would wonder if you'd take twin primarchs (i.e. like Alpharius and Omegon) scattered to Mars? one would remain there inducted into the Martian cult, another would go off on an explorator expedition around the same time as the unification war. They (at least the one that stays behind on mars) would likely be among the first primarchs rediscovered but thats up to you, could be cool that it isn't even realized they are primarchs until much later 
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Whether I'd accept it or not would probably depend on the sheet, but as a concept I could see it working, possibly with some tweaks, given we know the Emperor at least has the possibility to tavel between the worlds of the Solar system it's quite likely he'd have found them at a similar time to Alpharius, but it's an interesting enough dynamic if done correctly that we can discuss :)