I have absolutely no problem mixing designers. I just wear what I feel like wearing. In fact my bags and wallets rarely match. Today I'm carrying a Dolce and Gabbana bag with non-matching wallet and coin purse inside.
You can wear different designers at the same time....
Just as long- as only 1 of them is a signature fabric..The outcome of wearing more then 1 signature fabric at the same time is just a dizzing disaster IMO
If carrying a logo tote, then carry a plain small purse (could be the same designer, but in plain leather) or vice versa, but two same designer logo bags or even different designer logo bags together, just looks yuck, imo!!!