Packmaster Hunter and Mammoth Rider Synergy?

[AmbassadorZubat]

I am rolling up a few new characters to replace my samurai in my weekly game.

One concept that sticks out to me is a Half-Orc Packmaster Hunter (ACG) with the Beast Rider feat, a Triceratops, a Pteranodon and Boon Companion on both. This will add 6 levels to both of them (2 from Beast Rider, 4 from Boon Companion).

My question is at 10th level, when I start the Mammoth Rider progression, what happens to them? Do both my Triceratops AND Pteranodon grow to huge size, or just one of them? Do they both gain the benefits of Gigantic/Rugged Steed? Can I Rapid Straddle both of them?

[Markov Spiked Chain]

You can only choose a single creature as your Mount for Mammoth Rider, so the other one would remain normal size.

Huge Pteranodon from Beast Rider looks awesome though. I may need to figure out how to squeeze out a feat for Racial Heritage on my (human)Hunter.