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Like their legs are the same parts but they're assembled slightly differently so that as an ankle joint in robot mode Grotusque's ankle is set farther back, and as a hip joint in ??? mode the joint is lower on the body, and I think that works better than having the ankle more forward and the hip higher up like on Doublecross.
If I had more confidence in my pinning skills I'd probably try disassembling Doublecross' legs to try and make them like Grotusque's but I don't think it's worth fucking with the ankle joint of a heavy figure.