Carpentry and Woodworking - Half wall cap

I am finishing my basement and thought I would put wall caps(shelf boards) on top of the knee walls I have. Two of the walls are approx. 20' long and the aspen wood I am using only goes up to 12' length max. What would be the best way to join the pieces together?

A miter joint and some sanding should give decent results.
Do you plan to paint or stain?

I plan on painting the caps white. Maybe I phrased it wrong, but I need a way to connect pieces together to go the full length. I was thinking of using a biscuit joiner. Would this work?

When Marksr mentioned the miter joint, I'm pretty sure he was thinking of a "scarf joint" which is just 2 pieces cut at opposing 45 degree bevels, which overlap each other vertically, one on top of the other. These joints just need to be glued and pin nailed together to prevent movement. Even if the joint isn't perfect, a little filler and sanding makes the joint disappear completely before painting.

A scarf joint is always better than a butt joint, even if you use a biscuit. The 45 degree bevel gives you a greater surface area to glue and allows you to nail through both pieces at the same time. Butt joints have a greater tendency to expand and contract, breaking open the joint between them.

Ya, I meant a scarf joint. As a painter I don't always know the correct terminology for carpentry work:o

I've painted behind carpenters that have used both methods and the scarf joint always looks better than a butt joint

I love going back to a job I have trimmed after the painters leave. They make us look sooooooo good! Tip of the hat, Marksr.