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I'm not sure about Chase. For Citi, you'll have two separate accounts. AmEx will combine them in one account.
Not sure if there is any big difference.
I think for AmEx, you still have options to set up two login accounts. Or you can combine one and combine all MR points into just one account.
Since you can transfer UR points freely between accounts, there is not big difference. I'm not sure if you can combine Citi Thank you points across accounts.