As a wife-I haven't thought either of those things actually. I tend to be over-concerned for the other woman's needs-to the point where I end up screwing myself.
In one case, it wasn't a problem, because she was also that way-so we tended to be ultra considerate of each other.
But, in another case it was a freaking FIASCO! I ended up emotionally bereft and totally messed up because I wasn't ensuring that my own needs were taken into consideration also.
I think it really really depends on personalities.
Mind you-I also have little use for hte "institution of marriage" as a legal thing-and would welcome a "second wife" in our situation.
I just want her to be someone who treats me with consideration and family love and care the same depth as I do my chosen family. If it's OUR family-I feel WE should be equally committed, responsible and privileged.
For example; I wouldn't have any issue sharing the bed with a woman I wasn't sleeping with-(I would prefer for sex we take it to our own personal space),
but I would expect that she's going to participate in cleaning, maintenance, childcare etc.