As I suspected , nothing you can do, the only memory speed related setting is this one and it's on "Auto" by default, and you already know it's not set to 1333 due to memory being able to run at 1600 Speed with only the 4GB stick
But, even as I mentioned initially, if you could get them to run at same speed, timings, and voltages (which you can't adjust any of this), they likely would still cause errors due to 2GB/4GB mix and totally different internal IC chips.

Memory Frequency Limiter, VarStoreInfo (VarOffset/VarName): 0x35, VarStore: 0xC, QuestionId: 0xCF, Size: 2, Min: 0x0, Max 0x5, Step: 0x0 {05 A6 66 01 67 01 CF 00 0C 00 35 00 00 11 00 00 05 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00}
Default: DefaultId: 0x0, Value (16 bit): 0x0 {5B 0D 00 00 01 00 00 00 00 00 00 00 00}
Default: DefaultId: 0x1, Value (16 bit): 0x0 {5B 0D 01 00 01 00 00 00 00 00 00 00 00}
One Of Option: Auto, Value (16 bit): 0x0 {09 0E 9C 00 01 01 00 00 00 00 00 00 00 00}
One Of Option: 1067, Value (16 bit): 0x1 {09 0E 68 01 01 01 01 00 00 00 00 00 00 00}
One Of Option: 1333, Value (16 bit): 0x2 {09 0E 69 01 01 01 02 00 00 00 00 00 00 00}
One Of Option: 1600, Value (16 bit): 0x3 {09 0E 6A 01 01 01 03 00 00 00 00 00 00 00}
One Of Option: 1867, Value (16 bit): 0x4 {09 0E 6B 01 01 01 04 00 00 00 00 00 00 00}
One Of Option: 2133, Value (16 bit): 0x5 {09 0E 6C 01 01 01 05 00 00 00 00 00 00 00}

Okay dude. Thank you thank you so much! You are very awesome. May God bless you!