Hi DD,
I can only speak for myself, but I just can't get my head around being anything else but the one person I am. OK, I am a fairly girly guy in boy mode so I guess I don't differentiate between being in boy mode and being en femme that much (at least not in my head) so when I'm en femme or at work I am still me, it's just that I am feeling slightly different things about how I perceive myself (feeling either more or less feminine) ..... but I am still the same person - me!
My wife Marla and I made a promise to one another that when I was dressed I wouldn't "become someone else" ... there is no third person in our marriage. For us, it's worked out really well and whilst she doesn't call me Rachel in boy mode or anything (she calls me Angel) she does treat me the same whether I'm in boy mode or girl mode ... namely, she's very loving toward me. I should mention that my wife likes a feminine man and encourages me to express my feminine side in male mode quite a lot so it's easy for me to integrate the two sides into one.