My first reaction, here, is that the lag isn't going to be any less if you do something in SharePoint; if someone else checks it out, it's still unusable (well, unupdatable) until the other user checks it back in.
If what you want is really for two people to be able to update a worksheet at the same time, I was going to say there's no way to do that. The first contradiction that occurs to me is to put the data in Access instead of Excel—or in some other database tool, like Oracle or Focus or DB2. Of course, that's probably more trouble than you want to go to.
But come to think of it, I guess there's another way. If you have an agreement that you'll update some data and keep your cotton-picking hands off other locations in the workbook—if everyone has designated areas of responsibility, and you trust each other not to cross boundaries (or at least not very often)—I imagine you can each put your data in a worksheet of your very own, and construct another workbook that assembles the data.
To keep it simple, maybe you're going to update only row 1, and your partner only row 2, and the rest of the worksheet calculates values from those two rows. You keep a workbook on your machine (name it Row1.xlsx), and your partner has another workbook (Row2.xlsx). Then, in the main workbook (Main.xlsx), rows 1 and 2 have formulae that read the data in your two workbooks. When anyone opens Main.xlsx, it'll ask them whether they want to update the links; the user answers Yes, and it pulls the latest data from Row1.xlsx and Row2.xlsx.
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