Who takes the deductions?

I finished our taxes last week and delayed filing because I couldn't find a 1099 for interest and had to contact the bank. I was a bit ticked because every year in the past, married filing jointly, we got a refund. However, this year we have to pay, albeit a small amount. After everything was complete and entered, I thought of something that I didn't think of prior to entering everything into TT. I worked a long term temp assignment last year that brought our income up to the next bracket, but my income was considerably less than my husband's and was more pocket money for me. With my husband's income alone, we would have received a decent refund. When I input everything, I put all medical expenses under his income as well as charitable contributions. I am the one who had all the medical expenses for the year. Would it make a difference in the bottom line if the deductions were taken off my much smaller income even though we're filing married/joint? I realize we file married/jointly, but does it matter who takes the deductions in this regard or is it all lumped into one pot? I'm thinking the medical expenses allowable deduction percentage would be beneficial if it were off my income instead of his as well as the charity items we donated which include furniture and many household goods, thus possibly negating the amount owing and possibly giving a refund. Or is this the type of differentiation that married filing separate is for? I checked and with everything input as is, we would owe more money filing separately. However, would changing the medical expenses and taking into consideration they are more than the income percentage for my income alone be a solution? With my husband's income alone we would have received a significant refund even without the medical that I claimed on his end instead of mine. I am from Canada and new to US taxes. This is the first year where I have also had an income, so I'm not sure how this works. We always filed married/joint in the past.