I think your best option is to run a trapezoid or modified trapezoid course for the boards, and standard W/L for the cats. Here's what you need to do:

Look in the RRS, Appendix L. Set your marks per the diagram for the Trapezoid Course. Use 4 marks as shown plus the two start/finish line marks. You have the option to add gates if you have enough marks, but for just 10 boats per fleet, that is very optional.

Start the boards first, have them race this course: S-1-2-3-2-3-F. Have one of your mark boats anchor as the finish boat. To make it easier (and need one less mark), omit the separate finish mark and anchor the mark boat at mark 3 and have them finish there: S-1-2-3-2-3(F). Be sure to modify the diagram and include it in the SIs.

Start the cats next, and have them run normal multihull W/L courses, using mark 1 as A and mark 4 as C. Be sure to include this in the SIs.

Make sure marks 2 and 3 are far enough away from marks 1 and 4 so the laylines don't overlap. By doing this, you set the boards off and they will be completely off the cat course and there will be an extremely small chance of issues on the water.

Let me know if this makes sense, it would be easier if I were sitting in front of you with some salt shakers and sugar packets...

Mike