My boss/friend lost her husand and father within a day of each other. Is it proper for people to send one sympathy card or two?

Hi, drflock!

I'm sorry to hear of such losses in so short a period and I hope that you're doing okay! One card should do because of the short time frame between the losses. If there are to be two, separate funerals or memorial services, sending some flowers for each occasion would be a nice thing to do.

That is awful to here, but I agree one card would suffice.

One card is fine but be sure to acknowledge both people separately.
Not just a combined "sorry for your loss". If you know the husband or father, write some personal memory or thought of them.
Mainly just make yourself available to her. She is going to be hurting for a long time.

When my son was in the 8th grade one of his class mates died from cancer. I wrote a note to the mom (I did not know her) with my condolences and I did this periodically for the next couple of years. Like Mothers Day especially. I just wanted her to know that her son had not been forgotten by me and that she continued to be in my prayers. She eventually called me to say how much those notes had meant to her. You have to remember that right after a death they are bombarded with support, but their pain goes on for a long long time after that, and everyone else is back to their own life.