Yes, you can make the slot widerer by adjusting how far the slot cutter is below the router and cutting the slot again. I would suggest adjusting the slot cutter so it cuts the correct distance between the top of the wood and the groove. This will probably make the groove off center. Next you flip the piece of wood upside down and recut the groove. This will center the groove. If the groove is so wide that there is stuff left in the middle, just adjust the slot cutter's depth to match this stuff and cut again. Do this AFTER cutting the dual grooves on all pieces that need the wider slot though. Make sure to practice and get the fit right on a scrap piece first before you try on the real piece you want slotted.