<human>: Condense the text into a few key points

I read through the FAQ and did not see anything addressing this specifically. I *think* I know the answer, but just wanted to check with those more knowledgable than myself.

Background:
My wife and I each have a Roth IRA. I also have a Roth 401k, through my job, that I contribute to and they match up to 6% :). (My wife stays at home). We already have our emergency fund, and our only debt is our mortgage which we are overpaying on pretty heavily. We would like to set aside 15% of our income towards retirement based the Dave Ramsey plan. Currently we are setting aside 12% (6% me, 6% from my generous employer) - so we have decided another 3% is going to go into either or both of our IRAs. 

So my question is, *assuming an equal rate of return*, would there be any advantage to sticking all 3% into one account (say my wife's) vs. putting 1.5% in mine and 1.5% in hers. I understand diversity is important and that argument, but in reality the portfolios of the two accounts are pretty similar. I am just wondering about compounding interest, etc. I also understand that we have a maximum contribution per year, but this 3% wouldn't be enough to exceed that even if its all put into one account - so for now thats a non-issue.

I believe that it really doesn't matter, but just want to ensure there is not some mathematical  exponential growth that I am not thinking of/understanding that could have made alot more gains over the next 40-50. years
<bot>: Assuming equal rate of return, would be any different if an amount of money were put into 1 account, or if it were split between 2 accounts?